is generally accepted as the most accurate, VICE
as the most flexible, CCS64
as the most user friendly and micro64
could be described as the most anticipated. There's also Hyper64
which is an interesting intellectual exercise in making a really fast 64 emulator with little concern for accuracy.
As long as we're talking 64 emulators I have to plug Power64
which is very popular on the Mac. It isn't as accurate or feature-rich as VICE but it is very easy to use and has some really nifty features like the ability to cut and paste to/from the 64's BASIC editor.