That's a very loaded question...
Generally speaking, I think most would agree that NovaRossi is the best engine manufacturer when it comes to 1/8 buggy... But there is a trick
NovaRossi, Rex, and Top are the same engines! They just have different colored heads. NovaRossi is Purple, Rex is Blue, and Top is Black. I even think the JP mods are modded NovaRossis.
Next you need to decide what sort of power curve your looking for. Do you want low-end grunt for short tracks, or high-end scream for long tacks? or a little of Both? at this point I will agree with the previous post, the NovaRossi P5X is about the best for all-around performance... hard to go wrong. The NR 421B seems to be more top-end, but still good all around. The New NR 21-5 is supposedly the bees knees(did I just say that?
), but I have yet to speak to someone who has run it.
My next choices would probably be thus: Serio, OS, Werks, RB Concepts.