As someone who's been around the Sonic community for awhile, I can tell you the opinion isn't really THAT common. Sonic 2 is generally considered the strongest game in the franchise, both among regular old school gamers and old Sonic fans.
Sonic Adventure 2 tends to be the choice of the younger people who were introduced to it during the Dreamcast/Gamecube years.
Me? I'd probably give it to Sonic 3 & Knuckles, Sonic Colors, or Sonic Generations, depending on what criteria I'm going by.