Here goes, I'm gonna say what each Sonic era did best.
CLASSIC ERA: Sonic the Hedgehog (1991) through Sonic Triple Trouble (1995)
Peak Sonic gameplay
DORK AGE: Knuckles' Chaotix (1995) through Sonic R (1997)
Peak Sonic music
ADVENTURE ERA: Sonic Adventure (1998) through Sonic Advance 3 (2004)
Peak Sonic stories
DARK AGE: Shadow the Hedgehog (2005) through Sonic Chronicles: The Dark Brotherhood (2008)
Peak nothing
BOOST ERA: Sonic Unleashed (2008) to present
Peak Sonic writing (yeah, I said it)
CLASSIC ERA: Sonic the Hedgehog (1991) through Sonic Triple Trouble (1995)
Peak Sonic gameplay
DORK AGE: Knuckles' Chaotix (1995) through Sonic R (1997)
Peak Sonic music
Peak Sonic stories
DARK AGE: Shadow the Hedgehog (2005) through Sonic Chronicles: The Dark Brotherhood (2008)
Peak nothing
BOOST ERA: Sonic Unleashed (2008) to present
Peak Sonic writing (yeah, I said it)