It all depends on what you want to emulate. MAME if for arcade roms, but you can also get emulators for SNES, N64, Sega, Playstation 1,2,3, Xbox, Commodore 64, Commodore Amiga.
So to answer your question, MAME is the best for emulating arcade machines.
You can get it at from the official MAME website, easily found using google.