Sonic 2 for the Genesis if my favorite game of all time. So I’d suggest starting with that one. First Sonic was good too, but the second game refined the controls and made some small quality of life improvements. Then Sonic 3 + Knuckles and Sonic Mania are also excellent.

The 3D Sonics have been pretty hit or miss though. The controls are dumbed down and not nearly as tight as the classic 2D ones