Are you just looking for decent softsynths? If so, my favorite AU synth that I've found is Alphakanal's Automat. I haven't used it in a while since I moved over to the Windows side of things (and therefore DX and VST instead of AU) but I enjoyed it while I used it. A nice subtractive synth with lots of knobs to play with.
Version 1.0 is out and is freeware as far as I know, I used an earlier version which would not limit the output if you let the filter self-oscillate, so heads up there. I presume this has been fixed.
Hopefully this is useful to you. Best of luck.