There are two primary, actively developed C64 emulators, VICE and CCS64. Both are fantastic emulators and each have their own strengths and weaknesses.

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CCS
CCS has the advantage of being the most compatible and cycle-exact emulator. It runs pretty much anything you throw at it including the most complex demos and games. It's only available for DOS/Windows, though. CCS has better support for disk drive emulation, with the latest beta versions supporting bad GCR simulation and disk density.

VICE
There are two main advantages to VICE- it's open source and therefore portable, which means you can run it on lots of different computers, operating systems, and processors. It also emulates a lot of extra hardware such as ethernet adapters, RS232 adapters, etc.

You can add a simple "bad GCR" simulation patch to the latest VICE if you recompile it yourself, since it's not on by default. I added a simple patch way back to 1.14, but Wolfgang Moser added a better version to later sources. This allows VICE to work with games that check for bad GCR protections by randomizing the GCR read from the disk when it encounters $00 (zeroed) bytes in the G64 image. This is not exactly how a CBM drive works, but it's enough to pass the protections that need it and doesn't hurt real data.