If you are on a budget, I would recommend you try out the Sportwerks .21 engine (I know I will get flamed for this, but I believe in this oppinion). If you do not wish to try out this engine, then I would have to say the RB S7 II. The Fantom FR .21 is a good engine, and quite durable. But they can be a "joy" () to tune. The OS VZ-B would be the most reliable of the three in terms of the tuning-friendly carburetor that OS engines are notorious for.