I would put Vampire: Bloodlines near the top of my list also and Torment.  I loved Arcanum, magic, steampunk and sheep fucking.  I just don't know if I can name a favorite RPG, I love so many...I didn't like Baldur's gate or Neverwinter Nights as much as other people do, I liked them and all, but not as much as many other games.

My list of awesome RPGs  I'm sure there are more I could mention...

Arcanum

Vampire: Bloodlines

Planescape: Torment

Fallout, 1,2,3,NV

Ultima 4 Avatar

Drakensang the Dark Eye, and River of Time

Knights of the Old Republic 1&2

Dragon Age Origin, not so much the sequel

TES Morowind and Skyrim.  Oblivion was pretty good, but it's loot system made exploration pointless and I didn't have as much fun with it.

The Quest for Glory series.

I would have real troubles for chosing only one... But I think I'd give the medal to BG II. Not because it is absolutely the best (Torment is, from my point of view, as awesome... a bit more), but because it was the first RPG I played as a boy and I played, and played, and played... I can remember almost every single quest still So, that would be my choice.

Now, in the bunch of RPGs I have enjoyed after BGII, I would definitely have:

Vampire Masquerade: awesome, immersive, really, really cool adaptation of the pen and paper. 

Torment, of course... As cool as BG and even more imaginative, with an even cooler plot. I discovered it too late, but it really feels like reading a great fantasy novel.

Dragon Age: Origins, because it really recovered a lot of BG feeling, at least for me. Banters between followers, good plot, many choices... I loved it! The second, though, sucked completely. They even re-used maps without even editing the mini-map... Really confusing, really annoying.

Morrowind. Not the least because I was head of the mages before I remembered that there was a "main" quest

I once made a list of every game I've ever truly loved. You know, the games you'll remember (if you can still remember anything by then) when you're 90 and reminiscing about being young, hunched over your PC (or console) and experiencing virtual adventures. It was extremely difficult to narrow down the list and to decide between games in order to establish a favorite. But at the end of the day, I ended up having to concede that Morrowind was probably my most beloved game of all time. So I guess that makes it my favorite PC RPG of all time, too.

Skies of Arcadia was my runner up lol. (Yeah, yeah, I know. What can I say? That game captivated me.)

Amazing how much consensus there is, particularly given that some of the most beloved now weren't major commercial successes at the time (eg. Bloodlines, and Torment).

Though to really sort this out you need to decide if you mean "best for the time they were made," or "best right now for someone who's never played an CRPG." For example I loved the Nintendo game "dragon warrior" when I was a kid, but I can't say I'd really recommend it to anyone now.

I basically agree with Lord Xia's list, though I missed Arcanum and never really like any of the Ultima games.

Since MMO's weren't explicit excluded, I'd add in Dungeons and Dragons Online.  It suffers from the lame storytelling of other MMO, but really is a fun and well designed game to play with some friends for a bit.

Planescape: Torment. Only things that come close are Ultima VII, Morrowind and Betrayal at Krondor, in my opinion.

Let's not forget Temple of Elemental Evil with the Circle of Eight patch/mod. 

Baulders Gate

Neverwinter Nights 1

Knights of the Old Republic 1, and 2 (with the deleted content restored).

Mass Effect 1, and 2, but NOT 3.

Skyrim.

Baldurs Gate II a.k.a "We wanted to write a book but videogames sell better!" - Start in some prison and every time you click on another character there comes an A4 of text. And NO voice!     By the time I got to the first fight I have read for ATLEAST 5 minutes....move onwards, a fight starts! Yeeeejj     *3 sec later* One of my characters die....dammit....repeat 3 or 4 times....I FINALLY make it out of the dungeon but by then I realize I don't like nor understand the system and quit. I had it on 160% (is it speed or difficulty?)

This doesn't resemble any Baldur's Gate II I've played, and I've played it through several times. In the first five minutes, you get to release several characters from prison, equip them, discover a bit about them, and launch into a battle in perhaps the third room. Then, it's straight fighting aside from the eerie occasional areas where you find creatures trapped in liquid, discover a genie who gives you a quest, etc. If this is your idea of writing a book, maybe you should stick with a straight-up fight game.

Are you sure you aren't confusing it with Planescape: Torment? Because PS:T is very text-heavy. BG II gets right down to battles, after just a small amount of information. There might be a gameplay for it on Youtube you could check out.

This isn't intended as criticism of you. I just honestly think you're referring to a different game, and might enjoy BG II.

You're the best judge of what you like, in any case.