Flip a coin? ..except for the OS, which is a plain bearing (meaning bushing) engine. That means it uses bushings instead of ball bearings on the crankshaft, and I'd cross it off the list because of that.

OS makes a very good .46 sized engine that utilizes ball bearings, the .46 AX, but it costs quite a bit more. You DO pay more for an OS, but they are usually worth it at the same time. If you can afford the OS .46AX, I'd say that is the one to buy!

The one GMS you listed first has a high speed needle valve that is mounted directly on the carburetor, and while that probably IS the best place for it to be mounted, it requires your hand to be awfully close to the spinnning prop if you adjust it while the engine is running. It is also the least expensive of the bunch.

Tower and GMS engines are made at the same factory, and all of the ones listed are the desirable ABC type of construction, except for the OS LA.

Actually, there is one other difference between these engines. The Magnum weighs the least, according to the specs - 16.0 oz. WITH muffler. I'd probably go with the Magnum, because of that.

Really though, there are no WRONG choices here. I think all of those (the ones with ball bearings) listed are very good. Everyone will have their favorites, but it probably boils down to personal preference.