Oh, so now they have to be a SERIES?
Okay, umm...
1. Final Fantasy
In terms of sheer quality over an extended period of time, as well as both revolutionary and evolutionary aspects throughout that time, the Final Fantasy series has yet to let me down.
2. Mario
Same here, basically. There have been some average Mario games, but most of them are superb. The first three ever made are still the best, IMO, and luckily, I have them all in Super Mario All-Stars for the SNES.
3. Gran Turismo
The best racing simulators, period. Nothing even comes close to the realism displayed here, and the level of depth is also unmatched. GT 4 will continue the franchise reign, guaranteed.
4. Suikoden
Three installments, and all three are some of the best games I' ve ever played. They also sport the best-written stories of all time as well (the first two more than the third, though), and every last one of them has kept me playing through to the very end. 108 Stars of Destiny...I always want ' em all.
5. Metroid/Resident Evil (tie)
Between Metroid II: Return of Samus and Super Metroid, not to mention Metroid Prime, that franchise has been freakin' fantastic since its inception. It really doesn' t get much better in terms of action/adventure.
RE is just classic by now, and although they' re all a bit too similar, RE 4 will once again revolutionize the genre. It ain' t just a graphics upgrade, that' s for DARN sure. My favorites are RE 2 and RE: CV X, but they' re all great.
Also, just for the record, now that Splinter Cell is a franchise, I' d like to say that with a great third title, it will easily make my top 5. Grand Theft Auto will also easily make the list with GTA: SA; the first two atrocious installments are keeping it off for the time being.
Runners-Up:
Need for Speed
Sonic the Hedgehog
The Legend of Zelda
Castlevania (it has the very best side-scrolling 2D game in history, as well as several other awesome titles, but some earlier installments weren' t all that great)