Just to chip in with my picks-
Knights Of Legend. EGA old school (and early EGA, if I'm not mistaken) effort from Origins, but still one of the overall coolest RPGs ever. So many features in the game were ten years or more ahead of their time- the ability to completely customize your character pictures, the depth and breadth of the skill system, the non-linear storyline... While not the first game to feature completely open-ended gameplay (Wizard's Crown, Roadwar 2000/Europa, and the Phantasie series both spring to mind- possibly Wasteland, as well, although they were all within a few years of each other, so it's hard to tell), it did it beautifully, and the fact that certain character races and classes were unwelcome in some areas, while fine in others, really made you think when you created your party, weighing the natural advantages of a Blackguard or Barbarian, versus the inevitable discrimination you'd suffer.
Elite. Another oldie, but again, completely ahead of its time and possessed of a vision that no one could've reasonably expected to fulfill, at the time. What makes Elite so remarkable is that it did live up to said vision, completely. Be a pirate, be a trader, work for the military, mine asteroids, be a bounty hunter, hunt aliens- do anything you want, all while exploring several different, mindnumbingly vast galaxies, looking for the big score... and when you get your rating up to Deadly, watch as the novella from the manual suddenly becomes unexpectedly relevant. Arguably the greatest game of all time, IMO.
Morrowind. Not so old, but still an absolute classic, IMO. Great graphics, great world, builds on all the things that made Arena and Daggerfall so much fun, while avoiding a lot of the pitfalls they suffered from. The fact that you can add and/or create content so easily only makes a fantastic game even better.
Transylvania. C'mon- much like Oregon Trail, if you're in your 20s (or early 30s, in my case), and you've been using computers for a while, you've gotta know and love this game. Maybe not the best text adventure ever made, but still a lot of fun, in that old school, frustrating, "find the foozle" sort of way.
Darklands. In many ways a spiritual successor to Knights Of Legend (in my mind, at least), it still manages to be completely its own thing- a realistic medieval RPG. You can navigate by real-world maps of the period, your actions have consequences, you can work a mundane job to earn money to fund your adventuring, if need be, and if you go for the main storyline, you find yourself in a position where a pyrrhic victory may be your only choice. Deep, intelligent, and definitely a contender for best RPG of all time. Only really marred by bugginess issues, which took 7 patches to finally resolve if I remember correctly- but the game is so good, it was worth the patience and effort.
So, yeah- my additions, off the top of my head. Figured I'd just go in-depth about the ones I didn't notice mentioned before. Other, already mentioned games I'd call most memorable include Wasteland, Ultima III, Master Of Magic, and VGA Planets.
