KOTOR is definately a good one. I prefer the Xbox version though. Just something about it that seemed to fit better as a console game, but that is just my opinion.
I have never played this game before, but Deus Ex is supposed to be good. I think it is actually an RPG/FPS combination. Once again, I have never played it though.
Morrowind is also an alright choice (since it is probably around $20 now) and is the game before Oblivion. The graphics really are dated by today's standards, but some of the quests can still be fun.
Lastly, you can't go wrong with Oblivion (especially Xbox 360 version). Since launch, I have over 15 hours in it (that is A LOT for me over four days) and I feel overwhelmed with how quick the quests are piling up in my journal and I cannot seem to finish them quick enough (this is a good thing). I have barely touched on the main quest, just started quests for TWO of the guilds (there are like 5 or 6 with in depth quest lines), explored like six dungeons (over 200 according to Bethesda), and now I am trying to cure my vampire disease. That is just to give you a small idea of how much is in the game.
So I definately think that Oblivion is a GREAT choice for a first RPG. The learning curve is steeper than say KOTOR, but there is less "what now?" moments than in Morrowind (compass targetting helps A LOT in a good way). Sure there are a few gripes about it (I was actually worried about it because most games that I "read into" months before usually turn out bad), but it is the best RPG that I have played to date and I can seriously see myself putting over 100 hours into it (I am not hardcore enough for 1000+ hours). So if $50 is not super expensive for you, Oblivion is worth it.