NMP from Central hobbies seems to be the most widely used in pattern.

Although I have been running a 1650 NiMH 5 cell pack from Radical R/C, and I can't get it below 6 volts to save my life. Only weighs about 4.7 ozs with NO FIRE HAZARD. I have put 12 HARD 14-17 min flights on it, in high wind, flying advanced...and have more power than I know what to do with. That's with 6 JR digitals too. Anything more than 12 flights and I'm toast anyway.

But if you just MUST have a Li-Ion pack, I'd say NMP.

-Mike