For the budget conscious C64 user, I always recommend Jim Brain's uIEC/SD device. It has amazing support and has regular firmware updates to improve compatibility. It now even supports GEOS! The only more compatible adapter is the 1541U-II but it is much more expensive.
You can also get brand new IEC cables at Jim's store.
The uIEC/SD actually plugs into the C64 Disk Drive (IEC) port using a standard C64 disk drive cable and gets power through a second connector (that comes with the uIEC/SD) that plugs into the tape drive port. You control it using commands on the C64. There's also a file browser you can put in the root of the SD card that you can load when you first turn on the system that lets you use your arrow keys to navigate and select a floppy disk. If you don't have an IEC cable be sure to buy one from the same site.
To take full advantage of the uIEC you should also have some sort of DOS shell cartridge or add-on. I would suggest the JiffyDOS chip that installs inside the C64. You can get those from Jim as well.
I had the MMC Replay, which is an updated version of the MMC64. While the MMC64 and MMC Replay have a really nice, easy to use interface, I can guarantee that the uIEC/SD is going to be more compatible with the C64 games and software.
I'll be happy to help you figure out the best option for you, but I think the uIEC/SD with JiffyDOS installed in the C64 is the best kit if you don't want to spend the money on the 1541U-II.