Gothic II.
I played Oblivion first, but it was Gothic that made me love RPGs. It's my favourite for a number of reasons.
Atmosphere: it has a very deep atmosphere and a believable world. The characters are all quite believable (albeit with mediocre to often bad voice acting), many with their own goals and ideas that may or may not conflict with your own. There are plenty of characters who are just minding their own business as well, which makes the city and farms feel more alive. The music compliments the game as well, and helps build up the atmosphere.
Combat: Where to start. The combat is fucking brilliant in this. I personally think this has the best melee system of any game. Melee consists of timed parries, dodges and either single or combo-ed strikes in real time. Sounds simple when written, but it just works so fucking well in game. You can have awesome melee duels with NPCs and Orcs that can be very challenging but rewarding (I have a little tradition for myself where I fight an Orc one on one just a few levels into the game. Very tough, but fun and you feel accomplished when you finally beat him). I also like the desperation of combat when you're fighting more than one enemy at once. It's hard as hell, because you have to dodge and strafe to avoid the attacks, since you can't parry two strikes at the same time for obvious reasons. Also, you can parry blades and axes, but you can't parry magic or animals, so you need to master timed dodging to fight animals. You really have to be quite skilled to get good at the melee system, and that's something I appreciate.
The magic combat is also very good, with some fantastic spells. If you have the patience for it, magic can also be the most dominant force in the game. The variety of magic is great: there are summoning spells (such as the insanely powerful Army of Darkness), spells that freeze your enemy solid in ice - effectively making him a sitting duck until it wears off (usually by that point you have killed him, unless you use the spell to even the odds in battle) and there are also spells that cause amnesia on NPCs, causing them to forget any reasons they may have to hate you. There are a lot of more standard spells as well, but those are some of the more memorable ones. Oh yeah, there is transformation magic too, that allows you to transform into various different creatures (some of which are actually really fucking strong). It's just an awesome system.
Story: the story is nothing special by todays standards, sure, but it's very competent and it's what got me interested in magic and wizards (especially dark/demon magic). You don't really play a Gothic or Risen game for its story though.
The graphics are, of course, dated but it is a 2002 game so it's to be expected. I think they hold up rather well, personally.
Anyway, yeah, I love the game if this large post isn't evident enough. The only game that comes close to matching it in quality in my mind are the two Witcher games, though Gothic and Witcher are very different styles of RPG.
