@trekiej:
Depends on what you want and what your background is. I tend to go the route of the least resitance (stick with what you know ;-)). When we first got an A500 it came with AmigaBASIC, so it was pretty obvious to learn that, so I did. Once ACE BASIC became available I moved to that as it was similar to AmigaBASIC (same syntaxes) with a bonus of creating standalone executables that would run on any Amiga 65k OS.
AMOS is good for a quick and dirty program, downside is, atleast that's my opinion, that it doesn't default to Amiga's intuition components.