I have 7 characters, 2 DC, 2 AD and 3 EP.

I don't have any personal preference, but I make up a backstory and personality for my characters and then choose the alliance which makes most sense for them, usually based on where in the world they live. Even then it's usually not a deliberate choice so much as the end result of a series of events, or simply where they were born or grew up.

My khajiit templar left Elseweyr because of the Dominion but she was just heading away and ending up in Highrock was largely a coincidence. (I used a very similar backstory for my character in Skyrim, but she wasn't avoiding Cyrodiil entirely and so headed more directly north.)

My crafter did choose the Dominion, but even then it's because he's a retired soldier and wanted nothing at all to do with the war and a quiet place to live, and Summerset was the best choice available to him. (Even if he did have to live in Vulkhel Guard for years before finally getting permission to move to Alinor.)

Oh and my evil character (necromancer, vampire, thief, assassin, murderer etc.) decided when he was banished from Valenwood he still wanted to live among elves and thought the dunmer were more likely to have pragmatic attitudes and not totally over-react to every minor thing like his countrymen. (Yes I know the dunmer do not tolerate necromancy, vampirism and certainly not murder, but he didn't know that at the time. At least they're not going to freak out over a 'tree' which was barely even a sapling.)

Honestly if I had the option most of my characters wouldn't be part of any alliance (I know they couldn't go to Cyrodiil, they don't anyway so that's irrelevant), but since that's not an option I justify it by telling myself it's the local government assuming they support the alliance because they live there rather than any actual military affiliation.

EP/DC/AD

Because Skyshards, gotta catch them all
Also have Any Race/Any Alliance.

My main doesn't really have an alliance after seeing the horrors of war in Cyrodill and the ineptitude of others in the Pact he's doesn't support any alliance but no matter what he has too pick a side and has choosen to stay in the Ebonheart Pact my main is more of a mercenary or handy man that works for anyone with the coin to pay.

I wish there was a non aligned faction in game like Mercenary or Imperial Remnant being forced to pledge alligence to one faction or another is dumb.

Hello.....yellow(banana) elves and kitties......what else would you want!!!!

Because of loyalty

EP because they generally get me to 25k AP faster.

DC because
AD is run by a bunch of fascists with an ineffective queen (though i like her, and Raz is a bad ace and the khajit and bosmer are awesome)
EP is drunken racist braggarts backed by slaver scum (argonians are interesting lore wise but i just can't get into them as a player)

Plus Bill Nighy... Enough said.

Every TES game my first char is always a Redguard... and even though I had the Any Alliance Any Race I had no reason to change from the default DC at the time. But... having been ganked too many times by reds & yellows, I will never be making a character in either, nor changing any of mine to any other alliance.

Red because I mainly do everything with one toon, who started as a Nord, and then I realized what a poor choice I'd made and bought the any race/any alliance bundle so I could make him a Breton. Alliance Change tokens are a thing now but I don't see the point.

Some insight about my experience doing Cyrodiil on console: people here on the forums always talk about EP being full of zerging jerks, but I'm starting to think that's a PC thing and people take it for granted that everyone plays PC. The zerging jerks I've seen on PS4 NA are the blues. Every day they all get on and completely roll the map in all the campaigns. That's fine, I just wish so many of them weren't so smug and rude.

Oh, you're not wrong. I've seen enough back and forth finger pointing, everyone accuses the other alliances of the same behavior.

For me it may be a phenomenon of the time of day I play, perhaps? Any campaign I join, there's only a handful of EP on, and tons of DC. AD I honestly feel the worst for, they can't seem to get any headway.

Mostly EP. It was an arbitrary decision when first starting playing ESO & wanting to be in the same faction as my friends. I've PVP'd on the other factions briefly... but it's hard to not see blue & yellow as enemies now. As many people in this thread have already said.. all the factions have their zergs, 1vXers, tight small groups, bad players, good players & everything in -between. Every faction blames the other 2 faction for 'ganging up on us!' & every other thing that they think is wrong with the gameplay