I believe StormC is the best compiler, since it is still beeing developed. This is your choice if you intend to do serious development. SAS/C is also a pretty good one, but it is no longer mantained or updated.
Storm C 4.0 *IS* based in Gnu GCC, or so I read somewhere, and someone had uploaded the compiler source code that came with StormC 4 to Aminet (although I can't find it anymore).
GCC is one of the best compilers ever made, if you are entitled to CLI or cross-platform development. You can find at Geek Gadgets a newer version (v2.953 - released last year). GCC is used in allmost all platforms and consoles... Gameboy, GBA, PSX, PSX2, Dreamcast, handhelds... it generates fast code, and it tries to follow and implement the C++ standards and its changes.
*** update ***
I went to HAAGE & PARTNER and here is what they say:
26. February 2001:
GNU Sources of StormC
Our new StormC compiler system version 4 uses a specially adapted version of the GNU projects GCC and CVS. The source code for these projects can be downloaded from our server: GNU Sourcen (ca. 30 MB).
not bad for a lame unix compiler huh?