Wizards is a highly impure tribal deck that has very few reasons to be tribal. Patron Wizard and Voidmage Prodigy and Sage of Fables are about it (others exist but are even less playable), the latter is really quite poor and the other two are hardly power cards. The Patron and Voidmage are quite good at locking people out of the game but require you to get ahead on tempo first which can be very tricky in blue. There are wizards all over the place in every colour and while the tribal deck is always blue based it does often splash depending on the direction it is going in. Wizards usually ends up as some sort of cheap disruption based deck however I regard it as the biggest imposter in this top 8 list as the deck seems to be better without any synergy cards or creature type restrictions. These kinds of deck end up with most of the dorks in them being wizards, but with no useful synergies it is not worth making it a wizards deck. You are best off just playing the best cards and leaving the fact that most of your guys are wizards as an irrelevant coincidence. As a wizards deck it is the worst of the tribal decks but if it is just built normally as a tricksy tempo deck it would have better matchups across the whole metagame than half of the other tribal decks.
Merfolk is a pure deck that is separate to any other style of agro blue deck. It has no good stand alone merfolk but it has the best lords and the most lords which means you gain lots of momentum as you increase your merfolk count. Being blue you get to backup your dorks with counter magic should you wish and so a requirement to over extend is less problematic than it is for tribal archetypes like elves or goblins. There are several problems with blue agro decks in general which is why merfolk is so low on the list. Blue disruption for agro decks tends to cost you card advantage and is quite limited in application as well. In order to pack both disruption and some card advantage to make up for the disruption it becomes hard to fit in enough creatures. This problem is made worse by the fact that blue has a very thin selection of dorks to chose from which is made far thinner by restricting yourself to one tribe and thinner still by trying to get good tempo monsters where possible. Blue agro decks tend to be pretty good against control decks but really suffer against other agro decks which have much better tempo from their more efficient and reliable removal and bigger stand alone guys. There are a few ways to build merfolk decks, some use lots of free counter magic, others go heavier on equipment, some try to use things like Aquitect's Will to give your guys evasion.
Fearies is fairly pure but is also rather like the archetype Skies which is essentially flying dorks backed up with countermagic and other disruption. Faeries can be UB or mono blue quite comfortably. The black version tends to be better as it removes the issue of weak removal in blue and offers some extra high quality faeries to play around with. Black also adds cheap discard as another option for disruption. Another reason faeries feels less pure than something like merfolk is that there is very little synergy that requires the tribe type. Mistbind Clique, Spellstutter Sprite and Scion of Oona are basically it and the Scion is really quite weak and doesn't always make the final list. The Sprite is also playable without support from other faeries. Much like merfolk the deck is pretty solid against control and combo but struggles a bit against agro decks. Faeries plays less dorks than merfolk and so ends up with a higher average card power level in the deck, it also has a much higher curve with usually at least 3 dorks costing 4 or more.
Zombies is somewhat of another imposter deck and is really just mono black agro. There are lots of cards with zombie synergy however not all that many of them are any good, in addition to this there is, as with merfolk although not as bad, little depth in zombie creatures. I heard an interesting suggestion that some cubes used an imaginary "undead" creature type that included zombies, skeletons, ghouls and vampires. They then errated all effects that affected one of those types to affect them all. This would certainly give great depth to the tribe but it might make it too good, it is also more awkward for newer players to pick up. As with wizards the best zombie decks don't tend to bother ensuring all their dorks are zombies as there is little need, the only card that requires other zombies to be really good is Gravecrawler which is fine with a low count any way, I am a huge fan of Undead Warchief but compared to blacks other 4 drops he comes up looking very poor. Zombies simply doesn't have the quantity of good creatures to make curving out into a lord or two a better strategy than playing a better non zombie in the place of lords. Still, mono black agro is a sufficiently powerful and capable deck that remains mostly intact should you wish to swap out some good non zombies for some synergic ones to have some fun and nostaglia without getting crushed.
Soldiers reads much like the breakdown of zombies except for changing instances of mono black agro for white weenie. White weenie is good enough and contains enough soldiers any ways that you can take a few of the non soldiers out for weaker but more synergic soldiers and still have a pretty good deck. The various lords and cards that work with soldiers are a bit better than those available to zombies and soldiers has a vast vast selection of cards from which to build the deck. Many of the soldiers offer synergy in a non-soldier way such as Soltari Champion or Champion of the Parish (most soldiers are humans too usefully). Lords are less essential in soldiers as white has so many types of Crusade effect but there are quite a diverse selection ranging from two drops like Veteran Amorsmith and Auriok Steelshaper to high end one man armies like Captain of the Watch. These top end soldiers cost rather too much for a white weenie based deck to consider but with cards like Ballyrush Banneret and Preeminent Captain this can be eased. A cooler and more fun deck than white weenie but not exactly better as it suffers what most tribal decks do, a real dependence on keeping dorks in play.
3. Affinity (if you count this as a tribe)
Many wouldn't consider Affinity as a tribal deck to which I could pedantically argue that Artifact was the tribe and it is just a type rather than a subtype. Semantic arguments don't hold much ground as far as I am concerned and so I base my justification far more on how the deck feels to play and build, which is of course very like other tribal decks. You have to balance the number of guys and the number of artifacts carefully on top of more normal deck balances. Also with affinity there are many cards that are unplayable outside of the deck much like Goblin Warchief and Priest of Titania are outside of their tribal homes. These cards as primarily the modular and affinity creatures but others such as Ornithopter do feature in this group. Affinity is not the most powerful tribal deck but it is very close and has seen more A cube time than any other tribal deck as it has relatively few of the narrow cards that only go in that deck compared to goblins or elves. The rest of the deck is made up with other A cube mainstays which means including the archetype within in cube only costs you about 10 slots in your cube which is less than the other worthy tribal deck. Affinity has become much weaker since the changes to damage on the stack as well as my removal of the power from the cube. Moxes are good in all decks but they are unreasonably good in affinity. Even with these two changes weakening the deck it is still comfortably tier one and has brutal speed and tempo combined with impressive resistance to disruption. When the deck was in standard and block constructed formats I frequently recall situation where it was without cards in hand or in play in the mid game and would still come back and win...
Elves is quite like affinity in many ways as it very explosive and has lots of cards that are very weak on thier own yet very powerful in combination with the other cards in the deck. While more robust than affinity in some regards Elves is a less tenacious deck and is also much slower to build momentum initially and so is easier to disrupt. Elves is almost always an auto win against decks without some form of mass removal which is one of the reasons I have rated it above affinity despite affinity being more resistant to wrath effects in general. The real strength of elves is that they tend to give you mana and card advantage rather than making each other bigger or cheaper. This allows you to snowball out of control more like a combo deck than a beatdown deck. Elves can indeed be built as a combo deck or an agro deck and will likely only be 10 cards different at most. The beatdown versions focus on getting as much good stuff into play as fast as possible and swarming over for the win, often using Overrun. The combo versions focus more on drawing cards and setting up elf synergies but usually wins in much the same way. Comically the best dork in the deck is not an elf but an insect, Wirewood Symbiote is an outstanding card that has fantastic synergy with the deck. It gives protection to your guys, ramp and the ability to reuse certain effects. Elves has lots of redundancy in the good elves as well as a huge number of elves from which to choose. There are probably more elves that are playable outside of an elf deck than any other tribe and this means elves has the best selection of low curve dorks which is one of the biggest issues faced by other tribal decks. Do you play weak dorks like Sandbar Merfolk to make your curve look right or just suffer it and play only the best dorks?
Obviously goblins are still kings of the tribes, they have removal built in to goblins and so can easily afford to pack a much higher count than the other decks. There are an awful lot of goblins and most of them are awful, strength in numbers seems to work out well for them as the cream of the goblin crop is all of a very high power level and offer great synergy with each other. Cards like Goblin Lackey, Goblin Ringleader, Goblin Recruiter and Goblin Matron are all very weak on their own but work with each other to offer good value, consistency, tempo and card advantage. Like elves there is lots of redundancy in goblins however the curve is not quite as low which is one of the weaknesses of the deck. This can be offset by playing good quality cheap burn to sort out the tempo and curve or by adding things like Food Chain, Birthing Pod or Patriarchs Bidding which gives you the late game explosions that elves aims to do. Goblins destroys most other creature decks by virtue of having great removal latched onto the goblins complete with card advantage engines. It also is very good against combo as it is consistent and has a pretty fast clock when only lightly disrupted. Dedicated control decks are where it can struggle to win as the key spells can be denied and mass removal sorts out everything else. Despite this goblins has a good matchup against almost everything as it is just so streamlined and powerful rather like red deck wins. Its matchup vs elves is pretty comical as an active Sharpshooter is game over and that is a quick and easy thing to setup. Goblins can also afford a high curve as it has Skirk Prospector to add mana, Warchief to reduce costs and two kinds of Lackey to put things directly into play.