Well that's a no brainer - but before you choose a site, make sure you do your research into their payment practices:
a). Make sure above everything else, they don't shave!
b). Make sure they have IP *and* cookie tracking so that you get ALL your sales.
c). Find a site that has not only good conversion, but good retention
d). Find a site that has a good commission rate for product sales/signups.
MensNiche sites have all of the above, and we are showing awesome conversions right now, down as low as 1:25 for some affiliates, with payouts every two weeks and no minimums!
Hit me up on ICQ when you want to get started.