One of the most exciting parts of Magic is the flavor of all the [card Phelddagrif]interesting monsters[/card].
Whether it's the classic [card Monss goblin raiders]goblin[/card] or the elusive [card floral spuzzem]spuzzem[/card], every creature in the Magic universe has it's place. Some tribes are powerful enough to compete in Legacy, like [card goblin lackey]goblins[/card] or [card Lord of Atlantis]merfolk[/card], while others like [card Diregraf Captain]zombies[/card] or [card Ravenous Baloth]beasts[/card] have been powerful in Standard.
But which tribes are best for Commander? Since we just want to have fun, whatever tribe brings you the most enjoyment will be best for you, but it won't be very interesting if you lose every time!
Why Incorporate a Tribe in a Commander Deck?
A sleuth of [card Grizzly Fate]bears[/card] or a clowder of [card White Suns zenith]cats[/card] can be entertaining in its own right, even if the creatures don't interact with one another. I currently have a Toshiro Umezawa deck that takes advantage of many [card Pestilence Demon]demons[/card]. The problem is that there are very [card Blood speaker]few cards[/card] that actually benefit from having many demons in my deck, but it is fun none the less.
This is probably the best reason to include a tribe, because certain tribal cards work well together, which we can take even more advantage of by choosing a good Commander. Out of the many Commander decks I've built, one of my favorites was commanded by Horde of Notions. He alone provided enough reason to cram as many sweet elementals into my deck as I could.
Which Tribes Work Best
In other formats, tribal decks can be built using only the best 5-6 tribal cards available, because of the smaller deck size and maximum four copies of a single card. Commander decks don't have this luxury. It's a good idea to search gatherer for every creature of the tribe you are considering. Sometimes a tribe will seem fantastic but you soon find there are very few quality creatures.
Snakes is a good example. There are some sweet snakes, but very few [card Seshiro, the anointed]quality[/card] ones, so it would be difficult to make use of a snake theme. A blue/green snake deck led by Momir Vig, Simic Visionary may be viable though.
Higher costing creatures are usually better in Commander. This lets tribes that are weak in other formats to shine. Elementals, for example, work well because many have 'enters the battlefield' triggers, they span across all five colors, and are usually quite large. Second, Horde of Notions specifically takes advantage of an elemental theme, and lends himself to an awesome five-color elemental reanimator deck.
Card Advantage -
Another type of tribe that works well are those that can generate card advantage. A great example is wizards, led by Azami, Lady of Scrolls. Wizards have been around since the beginning so there are many to choose from, allowing for many different wizardly themed Commander decks. One caveat is their lack of threats, so be sure to back them up with a few [card Tidal Kraken]krakens[/card] or other juicy blue monsters. Selecting a different commander with [card Ertai, the corrupted]more colors[/card] would be another solution.
A second tribe that generates advantage well is zombies. There are many cards that [card lord of the undead]return zombies[/card] from your graveyard, which keeps the corpses forever marchin'. Using Grimgrin, Corpse-Born as the commander opens up endless synergy possibilities.
Which Tribes are Underpowered?
It almost seems like Commander is the opposite of most formats. Powerful strategies in Legacy, for example, are usually bad in Commander, and tribes are no exception. When other formats utilize a tribe they are usually taking advantage of its [card Goblin Piledriver]aggressive[/card] synergies, but this isn't very helpful in Commander.
Cheap and Aggressive
Naturally aggressive tribes should generally be avoided. Forty life, multiple players, and board [card Oblivion Stone]wraths[/card] are all reasons not to play cheap, aggressive creatures in Commander. Having seven goblins in play on turn five may be fun, but a well placed Wrath of God will put you right out of the game.
I've tried playing goblins, and seen many others try as well, but I have yet to see a goblin deck that can win consistently. Goblins are one of the most popular creature types in Magic, probably more numerous than any other creature, but they just don't provide enough advantage or power to stay in the game after a board sweep. There are even several enticing Commander options, Wort, the Raidmother, Wort, Boggart Auntie, and the newly created Krenko, Mob Boss. I really want these decks to be good, but I just haven't figured out how to build them yet.
Another aggressive tribe is elves. These are quite a bit better than goblins because they are primarily green (the best color in Commander) and synergize better. Ramping mana is a very powerful strategy and elves do it well.
Another popular tribe is soldiers, but they are very weak in my opinion. They are white (the worst color in Commander) and don't generate much advantage.
Not Enough Cards
There can be two problems here. One is that there are enough synergy cards, but not enough other creatures to support them. This brings us back to snakes. There are some sweet snakes, but not enough to use as a primary theme or even a sub-theme. The problem arises because the [card Sosukes summons]snakes that synergize[/card] with other snakes require a bunch of them, which makes it hard to take full advantage of these cards.
The other possible problem is that there aren't enough synergy cards. A good example of this would be giants. The are a bunch of [card inferno titan]sweet giants[/card], but very [card Sunrise Sovereign]few cards[/card] that synergize.
Why Commander is awesome
I'm not saying that you shouldn't use goblins, soldiers, merfolk, etc., themes in Commander, but rather that some tribes aren't as powerful as others. Commander is such a versatile format that almost any theme can be executed, if you're willing to support it with other powerful cards or sacrifice power altogether.
An example would be my Kiki-Jiki, Mirror Breaker deck. It runs a minor goblin theme restricted to [card Siege-gang Commander]goblins[/card] that work well with Kiki-Jiki. I have another deck led by Seton, Krosan Protector that uses druids as a primary theme. It isn't as powerful as an elf theme, but I like to be original with my decks.
Some tribes have enough cards to use as the main theme of a deck, while others can only be used as a sub-theme.
Examples of quality primary theme tribes: [card Horde of Notions]elementals[/card], [card Bladewing the Risen]dragons[/card], [card Azami, Lady of Scrolls]wizards[/card], [card Grimgrin, Corpse-born]zombies[/card] and [card Ezuri, Renegade Leader]elves[/card].
Example of quality sub-theme tribes: [card Krenko, Mob boss]goblins[/card], [card Sliver Overlord]slivers[/card] and [card Blood Speaker]demons[/card].
Other tribes that seem viable: [card Lin sivvi, defiant hero]rebels[/card], [card Iname as one]spirits[/card], [card Radiant, archangel]angels[/card], [card Ancestors Prophet]clerics[/card], [card Garza Zol plague queen]vampires[/card], [card Doran, the Siege Tower]treefolk[/card], [card Kangee, Aerie Keeper]birds[/card], [card Nemata, Grove Guardian]saprolings[/card], [card Oona, queen of the fae]faeries[/card] and [card contested cliffs]beasts[/card].
I am a proponent of having as much fan as you can in Commander, but for me to have fun I have to win sometimes. If making a bizarre [card cateran overlord]mercenary[/card] deck is what makes Commander fun for you, then do it, but for me I want my tribe to be able to crush my opponents.
When I create my Commander decks I am always sure to incorporate a major theme and a tribe is always a fun option. Sometimes tribes work well with other themes to create an interesting and dynamic deck. A great example would be [card gravecrawler]zombies[/card] and a [card Grave Pact]death[/card] [card Phyrexian altar]theme[/card]. However you use tribes in your deck (or even if you don't), just make sure you have fun with them.