I've used a bunch of different types, and they all seem to get the job done, so you might just go for the cheapest.

The only suggestion I'd make is that assuming you go for a rechargable, you also buy a cheap replaceable battery type, like [link=http://www2.towerhobbies.com/cgi-bin/wti0001p?&I=LXUM81&P=7]this one[/link], in case you run out of juice at the field.