I haven't used too many game titles under MorphOS yet, but I kind of enjoyed Fortis.
I have a copy of Quake III Arena for my PC (and I've always been impressed with that title), but I haven't been able to get it to run under MorphOS yet.
I also have a copy of Return to Castle Wolfenstein, but as I never got that far into it on a PC I haven't felt the urge to try it under MorphOS (great graphic on that one though).
One of these days I'll get around to one of the Doom ports as I always enjoyed that game.

I'd love to see some classic arcade ports and I'd like to check Mame to see if that's improved.

And, if we ever get JIT support for UAE, we may have enough horsepower to really enhance legacy game performance.