For C64 emulation MagiC64 is the best for 68K Amiga. Vice is slow. Frodo is another option, but it is not that fast. MagiC64 requires a keyfile, which is an obstacle, because you can no longer register it. Last and most unpopular option is the A64 package. It is incredibly fast for C64 emulator on Amiga, but the emulation is not accurate. It is good for BASIC programing and utilities, but for games it is no choise.
For Sinclair emulation one of the best is ZXAM - plays on window and is very fast, but emulates only the 48K Spectrum with AY chip. ASp is newer, more developed, emulates the 128K Spectrum but is slower. For the Sinclair if you need TRDOS emulation to play the newest games from Russia, you can try CBSpeccy, but it disables the multitasking and plays on fullscreen. Speculator is the fastest of them all, but does not have that many AREXX scripts as ZXAM.