Snes: Chrono Trigger is the best - no contest! (well, maybe Earthbound but I haven't played it

):
1) You can put in 12 hours into it and be done with it. HOWEVER you can also continue the game in New Game + and go on to 99 hours if you want to. That kind of flexiblity is something a lot of RPG games lacked before Chrono Trigger. In fact, I personally believe that Chrono Trigger invented New Game +!
2) Deep Story (Japanese Version) that makes you question the very essence of life and fate. In Chrono Trigger for Japan the main characters often discussed whether or not that they could change the future and if they really were meant for greatness.
3) Good Characters. Often too many times, games used the sterotypical heroes: the plucky 17 year old boy, the childhood friend of the main character (Love interest), the silent badass, and the friend of the main character. However, in Chrono Trigger, we have the silent 17 year old, his best friend whose a genius, a bold daring knight, a tomboy princess (love interest), the barbaric cave woman, the robot from the future who develops a soul, and a dark mage whose past is the main driving force for over 3/4 of the game. Only one of these characters know the main character from his childhood days. Other than Final Fantasy VI, we never had characters this deep.
4) Plot Twist that actually was more impacting then Aerith dying. When the main chracter dies, its game over. Chrono Trigger gave that the finger and kicked it to the curb. With the main character dead, you either could take on Lavos by yourself, or go on a quest to revive him. And it wasn't a worthless death, Chrono dies by sacrificing himself buying precious time to save his friends. It just wasn't him praying there and getting stabbed in the back. Nothing was ever this impacting.
5) MULTIPLE ENDINGS!!!!!!!! In a 16-bit game, you'd be lucky if there was even a 'Bad Ending' However, Chrono Trigger gave 15 possible endings, each with a scenaro for each of the Characters including Chrono. It was epic to see what would happen if you saved the world if Chrono was still dead, or if you recruitted Magus into your party.
It's hard to argue Custers Revenge as the worst though...
For PC: I'll go with Doom 2, and pretty much any Blizzard Game except WoW.
that's all I've got for now, but, Chrono Trigger just has to win, mainly because it was on the fucking SNES and it had ALL of that.