Anyone must choose the type of engine to use by the type of flying and type of airframe they intend to use. By asking which have the most power you miss the question which develop the most thrust. Large lumber spins up more thrust even with slightly lower rpm. Most high performance engines are designed to spin high rpm with smallish props. This may yield high HP numbers but may not hover a plane or make a large plane climb usefully.
In side exhaust engines I agree that Nelson and jett are on top but I've seen none at the club field.
Rossi, Webra, MVVS, and K&B make the most powerful .45-.50 engines I've seen at the local field.
I am constantly amazed at how well some engines perform while others just like them seem troublesome. I've never had an engine that I couldn't get to run acceptably.
If you plan to be in the hobby for a long time and want an engine that will be there too remember that parts availability is also important.

Strykaas: I have about the same number of planes as engines. I prefer to set them up and not move them from plane to plane. Too bad that I can't outfit them all with flightpacks at the same time. I am trying. I have an 1800 sq foot building in which I store models as well as two vans and a 6X10 enclosed trailer.