I believe Aztec C (by Manx) is a decent retro Amiga C compiler. It's not as popular as SAS/C, but it'll do the job. If you stick to example code from the era (circa 1990) you'll do fine.
The problem comes when you want to move on to more modern programming standards such as ANSI C which allows for more portable code. For that I'd suggest VBCC for C programming.
So the bottom line is: if you want to learn retro Amiga only programming, then Aztec C is fine. If you're looking for portability or more modern code, then I'd suggest VBCC or GCC if you're up for it.
