The Tau would be the easiest/most fun to play, I am borrowing a buddy of mines tau army for a campaign we are having, and kicking the crap out of marines delights me to no end. Now being on the recieving end is no fun with my IG and Nids, although the fun begins when my nids get into cc, yum. :x (I think I just ate too many fire warriors.)
Necrons aren't near as much fun to play as the Tau, not to mention as a first time 40k player, you don't want to have to worry about phase out.
As far as Eldar is concerned, I guess they would do well, I have never tried them out for myself, I just don't like them for some reason, never got beat by em, but they are just annoying.