OK so all the usual choices are mentioned OS,TT Pro-46, ST-51 etc. This is possibly the most hotly contested question on this site other than what second plane someone should get

Here are my favorited in a very particular order, in regards to a newer engine tuner:

Irvine-53 (Tons of power, runs on 5%, can use the Tower-46 muffler for extra rpm's and it is very hard to find anything negative about this engine on RCU-) I had this on an Ultra-Sport and it was a perfect match with unlimited vertical-

Webra-50GT (Same power as Irvine, maybe a little less torque, also runs on 5%, and is lighter than all engines mentioned previously)

OS-50SX (Again similar in power to the two above runs on 15%, is ringed for LOONG life, can take the Tower-46 muffler for increased perf. and idles like a swiss watch!)

I also love the 46FX/SF and TT Pro-46 engines. The ST-45 and 51 are great runners, and the Magnum is powerful but they seem to be hit and miss for quality, try a search on the Magnum-52XLS
One underestimated engine IMHO is the K&B-48 (Screamin' 48) it is a tad heavy but has great power and is made in the USA

Best performance would be a YS-45 and Performance Specialties muffler or Macs pipe -