The
MEGA65 C64 core has excellent compatibility with C64 software, and has been tested extensively. Someone in the forum even set up a testing rig to try a bunch of hardware-pushing games and demos on real hardware and the C64 core simultaneously to look for imperfections per frame, it was pretty impressive. It runs in PAL mode, and can generate a 60 Hz video signal with PAL dimensions over HDMI for modern displays that can't do 50 Hz. It can also do authentic 15 KHz RGB over VGA, which can connect to a vintage RGB monitor with an adapter cable. Configurable SID types, more display options, and REU emulation.
NTSC mode, IEC and expansion slot support, and loading CRT and PRG from SD card are on
the roadmap. I gather that this would make it more capable than the MiSTer core, though someone else can confirm that for me. (I've never tried to set up a MiSTer C64.)