Here's how I see it.

1)O.S. FX -- Great running engine. The one I had ran great without one problem with peeling. In fact, the fella I sold it to is still running it to this day with no problems. I think they're a bit pricey though (AX).
2)Mecoa -- Great bang for the buck. It's a remake of the O.S. SF series, which was know to be one of O.S.' best engines ever made. I am currently running the Mecoa, and it if it ever fails, parts are readily available. I've never had a single problem out of this engine from day one. One hell of an engine for $69.00 and true ABC.
3)Thunder Tiger -- Sounds to be a great engine, but parts seem harder to get than some other brands. I used to run their .61 PRO, and if the .46 is anything like it's bigger brother it's a good one. It's got a nice price tag at around $79.00
4)Magnum XLS -- Seen several of them run at my club. Seems to be a very good engine for the price $69.00. Same engine as a O.S. pretty much, only for the Magnum having the better liner (ABC) and a great deal cheaper.
5)YS -- Not even in the same league, price or performace wise, although a great engine.
6)GMS -- I've been told that it's the same engine as the Tower. If so, I want no part of it, the .46 that is. Heard, and seen, way too many problems with the Tower .46. If GMS is the same engine, well, you know.

I've had no experience with the rest of the engines mentioned previously in this thread.

John