Sonic R, Sonic Adventure DX, Sonic Adventure 2: Battle. Each has various qualities to them I dig about 3D Sonic. Sonic R has great physics and level design. I'd go into the time trial for hours as a kid to explore the levels...my first ever Sonic and my first game that I 100%ed. Interestingly, the versions of Sonic R I grew up with featured no music, so I never experienced the game with it's OST until much later...the game without music allowed me to soak in the atmosphere and sound effects much more than I would've otherwise. SADX, is what I wanted from the franchise after Sonic R: Sonic versing Eggman and other enemies, etc. with a storyline to it. It delivers in that regard. Sonic Adventure 2 Battle, while not as beloved to me as the previous two titles, still has a special place in my heart because of all the multiplayer goodness, and memorable music, etc.