Sonic Robo Blast 2 and Sonic Heroes.
SRB2 because of its mods (and soon, probably ESPECIALLY due to SA-Sonic, which may also make it also the best Sonic Adventure game
).
Sonic Heroes.. well, because of the gameplay and the graphics. However, the plot isn't too bad. In fact, I think it's good. About the gameplay: while I enjoy it sometimes, it is MORE enjoyable with the Sonic Adventure Physics Swap and the Physics Stage Fixes mod! It makes the game control more similar to the Adventure games and it also makes it MORE enjoyable.. even better than the GameCube version). About the graphics, those are
amazing. Especially Seaside Hill and Ocean Palace. I would gladly visit Ocean Palace IRL :). IMO, Final Fortress also looks cool visually, and I like the rails. But something I noticed with Heroes is that it doesn't even feel hard.. even with the 2 mods I've mentioned above. And not even with Team Dark, which is supposed to be one of the "Hard Modes" of Heroes, but from what I've played, it doesn't feel much harder?
Overall, it's a great game, even if some stuff are repetitive.
There's also SA2, but it's also a case of "a Sonic game that is rather enjoyed with a mod", which is sad. I enjoy it more with some visual mods and New Tricks, which removes the SpinDash delay and not only can remap the Light Dash, but even the Somersault and the Bounce! I also used to consider SADX one of my favorite Sonic games, however nowadays I enjoy it less than other Sonic games... Maybe if I change the visuals a bit by ReShade.. And no, not with Ray Tracing. With changing the colors.
I don't consider the 2D Sonic games my favorites, tbh. I like the 3D ones better, but I think that my favorite 2D Sonic game may be Sonic Rush. The visuals are good and the gameplay is fun! The level design is also decent.. even with those pits.