After having a surprisingly good time playing Sonic Dream Team late last year, I decided to replay Sonic Adventure 1 and 2 quite recently. While they are fun, they don't hold up all that well, they give me the same feeling that Super Mario 64 did me when I replayed it earlier this year; in the sense that I can definitely see the appeal of it, and how revolutionary they were for their time, they just don't hold up all that great.
On the other hand, Sonic 1, 2, 3&K, CD, and Mania are absolutely awesome to just pick up and play, especially when I only have thirty to sixty minutes to game. I'd say 2D for sure, while the 3D entries are a mixed bag, decent at best, absolute dog crap at worst.