I'm on OS X. I often like to use
Virtual 64, which was most recently updated in 2019. It requires Commodore 64 ROM files to run, which you can grab
elsewhere. The ROMs are not part of the emulator package due to copyright restrictions.
I like this emulator mainly because it's so drop-dead simple. It wasn't targeted towards engineers to figure out how to install the thing just to play a quick game of something. If you just want to run typical C64 files (d64, prg, etc.) it does the job very well.
Professor Hoffmann:
The VirtualC64 project is based on three major goals. First of all, I am trying to achieve high compatibility by emulating all components cycle accurately. Secondly, I want to create a fun-to-use emulator that presents itself as user-friendly as it can get. Last but not least I am trying to maintain a well-documented and easy-to-read codebase that makes it easy to add new features. If you agree with VirtualC64's philosophy, you are welcome to join the project. The core emulator is written in C++ and the graphical user interface is a native Cocoa application written in Objective-C. Please note that I don't want to port the emulator to any other operating system yet. VirtualC64 is open source software published under the GNU general public license.
I also use
VICE for OS X. It can emulate virtually everything you throw at it: C128 mode, PET, VIC20, NTSC/PAL, various disk drive formats - you name it - and it works great. The amount of work and effort put into VICE is nothing short of inspirational. For the most variety and options, VICE is the bomb.