Part of the answer depends on what class you are flying.

If you're flying SSC, you are limite to $60 for the engine, making the Magnum .15, the Megatech .15 and the OS .15LA the most common choices (not necessarly in that order).

If you're flying A class or something else, the OS CV-A is hard to beat, though a tad expensive. But it is more reliable and has great power.