What a general question!! I'll give you a general answer...the RB C6BBT7

Of course, there are many factors to consider...budget, what kind of track it will be used on (if racing at all). If not racing, then you could go for a bigger engine, but you won't see any gains over a C6 in a buggy. The bigger engines are better suited to monsters and truggies.

So...

1) How much you want to spend?
2) What's more important to you, acceleration or top speed?
3) Starting method (top engines REQUIRE starter boxes usually)
3) Racing or Bashing?
4) If racing, what kind of track?

That will give us better info to steer you in the right direction...