SAS/C 6.58 generates pretty good code. You can even run it on your Mac or PC using AmiTools if cross-development is a must. It has basic C++ support, but no C99.
VBCC is pretty ok, very simple to use and set up. It has C99 support but no C++. It's not meant for 68020+ only, the source code is available so you can compile it to run on an A1000 if you want, or any other computer and operating system for that matter.
Then there's GCC which is very powerful, but has always been a mess to set up and it eats more RAM and CPU.
