I enjoyed Sonic Adventure 1 & 2. I didn't think they were perfect when they came out, but I spent lots of time with them, only cringing at points. (I never though Big's levels were that annoying, BTW. Knuckles', however....)
I don't think the first one stands the test of time--and certainly the GC port made things worse. I still have fond memories of the second one, but have no pressing desire to relive its love-hate duality in Sonic Unleashed.
Sonic 2 and Sonic & 3 Knuckles are still the most timeless entries. Everything since Sonic Adventure 2, besides the 2D handheld entries, has been bad or worse, though.