Regular black 3DS; I wish I had waited longer before buying it. The only other colour that was out at the time I bought was the teal but my brother had it and I didn't want the same as him, heh.
Originally, I had a half teal, half pink 3DS*... but the charge port broke. v-v Thankfully, I was able to transfer the data when I got the pink one with a capture card.
*I got my teal one during the NYC launch and then they launched the pink one in December and so I decided to do half and half essentially because I still liked my teal one and my friend customized the top for my birthday, but really wanted a pink one.
I play New Leaf on my regular sized, white 3DS. Never really care for the colour or pattern anymore, since I'm looking at the screen and not the front of it.
I play on the normal red 3DS and I've had it for a year and a half now ヽ(?▽`)/ although I'm thinking of adding a skin sticker to it to liven it up a bit.