Aoki, "Aoki-sensei" Daisuke from Kodomo no Jikan. Reasons of which I'd rather not get into. I'd let to keep my reputation relatively clean.
Of not him than Seiji Sawamura from Midori no Hibi. I've always dreamed of having a cute loli as my right hand. (if this sentence doesn't make any sense to you go read the plot summery on wikipedia or something.)
And lastly, I think being Hayate from Hayate no Gotoku would be pretty awesome. I'd be a badass and would already have a lover (And a cute one at that). And the fact that I've already read about what will happen to me is helpful. I'd destroy that stupid robot the first time around so I wouldn't have to fight it like twenty billion times.
I could really go on for a lot longer since I think threads like these are fun. But I'll keep it at 3 for now.