It was the Dojo. When I first saw him being released, I was all "Oh man, he looks like he'll be weird to play... I bet I'd be the only one playing him in my area!" Beyond that, he was always my favorite character in Donkey Kong 64 (still one of my favorite games evar), and the comment that Teneban quoted about his play style being weird sounded like it'd be a fun challenge to overcome. Then came... Wolf. I saw the videos of the one guy showing off each character's movesets from the Japanese version of Brawl, and I remembered how much he was raving about Wolf being the new Fox. Well, I had mained Fox in Melee, so I was thinking "It's like Fox, only better!" I was wrong. After about six months of 2500+ SDs with Wolf alone, I remembered how I had wanted to play Diddy and tried him out again. He was really fun, and in the same vein as Teneban I wasn't using Bananas very much at the beginning; heck, I wasn't doing much of anything worthwhile at the beginning, other than having fun with his moveset x) Once I came back on Smashboards and finally came to the Diddy forums (per DanGR's suggestion) and I learned about/started practicing all the ATs, I fell in love with the chimp. Some time spent trying to pick up Falco as a secondary has gotten me BPG-happy, and time with Wolf and Kirby has made me spend probably a little too much time in the air, trying to space a F-Air that isn't as long as I keep thinking it is xD But I love his Banana combos, I love the fact that I can and have to get better every time I play (Diddy has so much room to grow at the level I'm at), and I like the fact that I was able to contribute to the community in some way for all the help it gave me in playing our monkey
So why did I pick Diddy? He's fun, Sakurai said he would be, I loved him from DK64, and Wolf sucks xD
EDIT: Not to take away from this current topic (I want to see everybody's answers!), but has anyone else noticed just how much the other character mains seem to hate us? Almost everybody I've played recently has something to say along the lines of "I hate Diddy, I hate the Bananas, I hate his playstyle, I hate the way he moves, I hate the sounds he makes, I hate everything about him, and I hate people who play him"... No joke O.o (That's almost an exact quote from someone standing behind me at a tournament during one of my pools matches)