Ultima games is my favorite RPG series of all time, it is way ahead of its time, it bypasses the "beat the boss and save the world" formula, instead it focuses on moral choices and a huge open world where you can do pretty much whatever you wanted (you can forge your own sword and bake bread even!), even the big open world RPGs of today (Oblivion and Fallout 3) didn't surpass it.
The expansion pack for Ultima VII is every bit as good as the original, but it transported you from the familiar Britannia to Serpent Isle, it made some minor changes from Ultima VII, otherwise, it plays very similar to Ultima VII.
I loved "Wasteland" back in the late 80's, this game is the spiritual successor to that game. So instead of the generic fantasy setting, you are in a post apocalyptic west coast, and it has a great atmosphere and a very fun and strategic combat system.
This is the game that made me upgrade my old PC XT to a 386! It has great graphics (for its day), and a huge open world (trade mark of the Ultima series), it spawned two other "World of Ultima" games, "Savage Empire" and "Martian Dreams", both were excellent as well.
I have played final fantasy series since final fantasy 1 (all the games releases in the US, so 2, 3, 5 are not included), most people think of Final Fantasy VII when they think about this series, but for me, FFX is the one I liked above all other Final Fantasies.
Bioware exploded onto the PC RPG scene with Baldur's Gate, and it couldn't come at a better time, it featured a rich world (based on Forgotten Realms), and have a great story and memorable characters (Boo!)