For me, I have really enjoy those fan games that are a full on experience. Some of the biggest I have played are of course the juggernaut that is Sonic Robo Blast 2 which has an entire campaign, various modes, various characters, unlockables, an outstanding mod community, online gaming support, and a stellar soundtrack. Other fan games that made me feel that Sonic vibe I always want in the main games are Sonic P-06, Sonic GT, Sonic Project Hero, Sonic 3 A.I.R., Sonic Before the Sequel (Have yet to play After the Sequel), Sonic Beyond the Speed, Sonic 3D in 2D, Sonic.exe The Assault Episode 1 and Sonic vs Darkness. There are more that I have indeed played but that would take even more time. I am eager to continue to play more fan games and updates of already existing fan games as the Sonic mod and fan game community is off the charts and very talented and creative.
Sonic 2020 brought me a believe in good Sonic game with a style of modern games. I was amased by it and also kinda a master of it, i even made some speedruns.
Sonic GT also amased me at the time, and again, i mastered Shadow there, i even made a speedrun which were the world record for some months.
Also, Sonic Before The Sequel, Sonic Robo Blast 2, Sonic vs. Darkness, Sonic Dimensions were more or less fun, but didn't cause me the same emotions as previous 3.