Shadow the Hedgehog and Sonic 2006.
The former is an absolutely atrocious abomination that should burn in the deepest, darkest, hottest depths of Hell.
The latter, I don't think it's near as bad as everyone says, but it was definitely a disappointment and considered to be the last straw for many old fans.
Then we got many years of average (Rush Adventure, Unleashed), mediocre (Black Knight, Rivals, and Dark Brotherhood), and abysmal (SECRET RINGS!!!) games before we got Colors and Generations.
Hardware Comparison Threads:
Current Thread
Is Hardware Getting TOO Powerful?
Older Threads:
PlayStation/Xbox/Switch: 2022 Edition
PlayStation/Xbox/Switch Hardware Battle: 2021 Edition!
PlayStation 4/Xbox One/Nintendo Switch: 2019 vs. 2020
PlayStation 4/Xbox One/Nintendo Switch: 2018 vs. 2019
PlayStation 4/Xbox One/Nintendo Switch: 2017 vs. 2018
PlayStation 4: 2015 vs. 2016 vs. 2017