Unpopular opinion, but Sonic the Hedgehog 2006 is now my favorite 3D Sonic video game. After seeing what the fan community has done, I see the enormous potential it had. I loved the music, the combination of the speed lanes of Adventure 2 with the open-ended nature of the first Adventure title, the character/enemy designs, the stages, and the Final Fantasy Esque storyline. Sadly though, it was released too early due to the executive incompetence at SEGA.
If it was given enough time in the oven so to speak, Sonic the Hedgehog 2006 might have been as well-loved as the Adventure series.