The Blue/Black XL. There's a bit of a story here though.
Last year I originally bought the regular size Aqua Blue one, but within just a couple days and not even owning a game yet the home button broke and refused to work, so I returned it and got my money back. Upon some research I found out that the XLs have better built Start Select and Home buttons.
So I searched online on some local classifieds and found someone selling a Blue/Black XL that was brand new unopened for the price of a regular size 3DS. Needless to say I bought it as Blue is my fave colour and it was a great deal. I have also had absolutely no problems with it and I've had it since July 12th last year. (The same day I got Animal Crossing)
I originally had a red standard 3DS which I sold to my brother later so I could get a red XL, though I still use the standard one for trading Pokemon (because Bank is stupid for not letting you transfer eggs on it...)