Sonic is definitely a contender. they have enough new fangames out each year that rhey have a yearly festival, SAGE (Sonic Amateur Games Expo). some of them approach the quality of the classic games and have become classics in their own right, like Before the Sequel and After the Sequel. many of those devs have gone on to be successful indie gamedevs

Christian Whitehead is the developer of a fangame called Retro Sonic, and at one point pitched a port of Sonic CD to (then) modern platforms using the engine he built for it. SEGA accepted the pitch because the engine was so accurate (notable considering Sonic Team had recently failed at their own attempt and mismanaged another team’s) and the port was so well received that he was hired again to port 1 and 2. those went so well that he was hired to do a new game and was able to hire a bunch of fangame scene alumni to create Sonic Mania, which was the best recieved official Sonic game in a long, long time. whether or not the Sonic scene has the best fangames, they most certainly had the most impactful on their respective official game series