I like Gambit.

Gambit's one of the fastest Scheme implementations and it plays well with C: it's portable, can embed C, and can compile to C. Any platform with an ANSI C compiler will run your code.

Gambit is stable, has an excellent debugger, and has lightweight threads. The threads have attracted some interest; see Termite.

There are a couple drawbacks:

The first is Gambit's mark-and-sweep GC; it has no generations, it's not incremental, and it's not parallel or concurrent. For the vast majority this isn't something to worry about yet.

The second is the big problem: a dearth of libraries. Library availability trumps almost everything else. PLT at least has some.

If you're planning to write a commercial game, Gambit is a good choice, even with the GC. If you're planning almost anything else, PLT is saner.