I'm gonna piss a lot of fanboys/girls off here and say Mario.
Frankly, I've dislike Mario as a franchise from the very beginning, and that's not just because I was a SEGA-kid. I must admit I do like the old games (up to Super Mario World, and MAYBE '64), but the later games, and especially the newer ones just seem to have become a parody of itself. I've noticed that the series has added nearly nothing to itself as it's progressed. The last 3 major releases in the series have been nothing but the same, tired old money-spinning exercises by Nintendo, shamelessly flogging the poor plumber's skeletal corpse; Galaxy 2 is a rehashing of Galaxy 1 (obviously), *New* Super Mario Bros. Wii (by far the most preposterous and pompous game title I've ever heard) was just a multiplayer remake of the first Mario, and Galaxy 1 was just Sunshine with an extremely slight graphical improvement.
That being said, Sonic hasn't fared too well either. Ever since Adventure it's really slipped in terms of quality, though at least they've tried new things regardless of whether or not they turned out well for them.
Other than those glaring examples of VG franchises not ageing well, I'd say GTA really took a step down the ladder with GTA IV, and I couldn't help but get the feeling Rockstar were reluctant to announce GTA 5.
Rayman really slipped off the radar, but that's only because Ubisoft bizarrely decided to stop making Rayman games with Rayman in them (fingers crossed that Origins is good). Crash still has some life left in him, though not as much as he used to. I haven't played Spyro since the PS1 era, so I can't really comment on that. And finally I think the Tony Hawk games are on their last legs.