Depends on how much you want to spend. I just got the Polar s410 with the Wearlink transmitter and like it pretty well so far. $210 on lots of websites. You might also look in the Gear forum. Lots of HRM stuff there.
A basic one that I recommend is the Polar F4 $90 - does max HR, average, time, and approximate kcals. I really do like it! It has a smaller wristwatch made for women.
I tried a Mio strapless HRM - don't recommend it becaise you have to stop exercise in order to take your HR. It's not very accurate either.
Got my Polar 725x delivered 30 minutes ago by the UPS man ($339 at Colorado Cyclist). Good thing because I'm heading out the door in a few minutes to travel for a 50k ski race tomorrow and I want to try it out. My old Acumen monitor died. You should decide whether or not you want computer download capability. Go to www.polarusa.com to see their whole line of monitors. Good luck.