WinUAE is the king of Amiga emulation. VICE the best 8bit Commodore emulator. I tend to believe Hexc64 is also better than MESS for C64. These two are very far from current MESS emulation for the respective systems.
Spectrum has some good candidates like (stupidly commercial) Spectaculator, or I also like EmuzWin and ZXSpin (0.7s is the newest - still old - I found though). SainT and blueMSX (which is not just for MSX) are also interesting emulators.
Still MESS is fantastic for what it does (and a possible merge with MAME in all respects would be lovely too). Systems can get better and better and some modules "help" other modules sometimes.