From Sonic Retro
Ranking Attack crash
On the Steam/Win32 version, attempting to use Ranking Attack mode will usually result in the game crashing. This crash is known to happen if the player has more than 100 friends on Steam. This makes it nearly impossible to obtain the "Time Attacker" achievement.
There have been reports that the crash does not occur when running Sonic Generations on Linux using WINE.
This crash does not occur on the 360 or PS3 versions.
Fixed in a patch to Steam by Valve.