First playthrough (which was also my first experience with an Elder Scrolls game), I chose a Breton simply for the magic resistance.
Second playthrough was a Nord, because I wanted to do the civil war questline on the Stormcloak's side, and thought a Nord would be the best fit, roleplay-wise. I came up with a lot of backstory for her, and she's still my favorite character.
Third playthrough is a Bosmer, because I wanted to play a pure archer and had this vision of a Legolas-type romantic woodland elf that I thought a Bosmer would fit perfectly with.
Fourth playthrough is a Dunmer, again for mainly roleplay reasons. I want this character to be an evil bastard. Dark Brotherhood assassin, "Mythic Dawn cultist" (i.e. do all the Daedra quests, and specialize in conjuring Dremora, probably only wear Mythic Dawn robes), pyromancer. I haven't used destruction magic much since the early levels of my first game, so I really want to focus on magic this time around, and I always knew I'd choose a Dunmer for that.
I've also tried an Orc, who only got to level 15 or so because I didn't really care for him; if/when I play an Orc again, I'll use a much smarter build. And an Imperial, who was supposed to be a swordsman who refused to do crafting of any sort because he was a "nobleman" and crafting was "beneath him," but he only got to level 8 before I accidentally saved over his only saved game, OOPS. I'll be trying him again someday.
Also have characters and builds in mind for future Redguard and Khajiit playthroughs.
Basically, I want to PLAY ALL THE THINGS.