XL mod is the one I'm currently most involved with, mostly because the GA mode works well, but BKB is also a super quality piece of work. There's a beta patch for it that supposedly fixes GA mode, which I expect to try soon. My experience with Med mod is much more limited, but it also has many fans and has garnered high praise.

I don't think you could go wrong with any of them, it just depends on your favorite style of play. Check out the mod forums and read up on them, I'm sure something will suggest itself to you.