Out of all the Amiga's I've owned (A500,A1200, A2000, A3000, A4000, CD32) I have to say, I like the A3000 the best. Good build quality (but not ugly like the 2000), nice SCSI, built in VGA Support and UNIX to boot. Does everything I want really well.
I found the A4000 build quality to be shoddy and they seem to be a bit flakey on reliability. If you had to have an AGA machine, I'd go for a 1200, since people charge INSANE prices for A4000s on Ebay.