Morrowind is definitely the one RPG that blows all the others away, but it depends if you can handle the vastness and the liberty you're being given. Many ppl feel lost when they start Morrowind...like "ok, wtf do I need to do?"; because the game really allows you to ignore the main quest altogether and live a life outside of it.
Erh, anyway, Morrowind + Tribunal+Bloodmoon (its expansions) should keep you busy for weeks, if not months.
More "on-rail" RPG's are Dungeon Siege II, Fable, Vampire: Bloodlines, Sat Wars: KoToR II. From those I recommend Bloodlines for its atmosphere, but it is really heavy on the ole' machine and needs serious patching. Kotor II is a very big 'maybe', if they patched it a lot by now. It was positively riddled with bugs.
Then there's the older RPG's that still pwn j00. Baldur's Gate II + Throne of Bhaal still make for great gaming even today...there's Icewind Dale II which is very enjoyable and last but definitely not least: Kotor I. All of these should be really cheap by now and are top quality. From these I recommend Kotor I because it's one of the best storylines I've ever seen in a game and the graphics are still fairly recent.
So, to summarize: Morrowind and expansions for long and intense immersion fun. Bloodlines if you want a graphically speaking superb RPG with a good atmosphere and don't mind installing some patches first; and have a monster pc. Kotor I for solid quality entertainment RPG in the Star Wars universe (that should be a plus for you
).
All of these three are on budget lines, so you won't even have to spend a lot of cash.
Let us know what you went for.