Morrowind
Said why to death here and elsewhere. The ultimate game just to get lost in. Spent years of my life modding the thing - anyone ever play the epic quest
Muffinwind? - and don't regret a minute of it.
Fallout 1
Released alongside Baldur's Gate, this is the game subsequent RPGs should have followed. A fantastic world built around real, tangible choices and consequences, no game before or since has allowed the player to define and play a character to his strengths in the way Fallout did. Add Jagged Alliance 2-style combat and you would have an utterly perfect game.
Planescape: Torment
I don't play games for the story. I used to, then I started getting tired with the frankly childish writing and plots in 90% of "great story" games, whose fans seem unable to separate quality and quantity of writing. Then Planescape came along. I've only truly enjoyed one or two game stories since (Mask of the Betrayer), and it's because this beauty spoiled me for life. Gives most good novels a run for their money, and almost always integrates the great backstory into the game world and sidequests and makes you work for every scrap of information - not just that which will lead you to your goal. No matter how many times I replay, there's always something new to discover that sheds new light on the mystery of the Nameless One.
Jagged Alliance 2
Hairs will split over whether it's an RPG or not, but I've seen no good argument as to why it isn't. And it has the best damn RPG combat ever made.
Might and Magic 3-5
Back in the day, RPGs were about a very simple formula. Explore. Kill. Get stronger. Find loot. Venture into new, forbidden locations that you have no business being in, just to steal some obsidian weapons and flee before the dragons roast you. Exploit the game and AI just to stand a chance of survival. Watch those dozens of beautiful little numbers go up, and up, and up. Gaze at exotic locales and commit genocide on the entire local population. RPGs may have become a confused and schizophrenic beast from then on out - even my beloved Fallout and Morrowind were considerably warped takes on the genre, though arguably improvements - with the genre now a sad limpet-like thing stuck to the side of glorified dating sims and shooters, but for a taste of The Way Wot It Once Were, you can't do much better than grabbing the M&M 1-6 pack off GOG.
Can't pick just one, but if I had to on pain of death, I'd die screaming "Fallowindaaargh".
This post has been edited by POOPOO MCBUMFACE: 21 May 2011 - 03:26 PM