There's an ample supply of both. If you're asking about the ones listed in this thread, though, most are intended for a single player.

I have NWN Gold, and I really would like to play it....but it's just not very interesting. At all. IMO theres really no reason to play it, it has much of a narrative as World of Warcraft, and for a single-player RPG, that's just BAD. The combat mechanics are kind of cool, and the way the combat works is pretty cool, but taking on more than one monster at a time after the starting area is quite difficult. You're better off playing something else.

I take your point, but to be clear, NWN OC is the worst game I ever completed. That's a big difference between that and worst RPG I ever played.
The Diablo comparison is the waves of meaningless cookie-cutter enemies coming at you followed by a 'go-and-fetch' quest; rinse and repeat. In Diablo, that's enjoyable. In NWN, its frustratingly dull, considering they spawn out of mid-air and generally have no AI worthy of the name.

Oh yeah, those are excellent.
Also everything by Stefan Gagne, starting with Elegia Eternum and the Penultima series.

As far as MotB goes, the balancing was horrendous. Spirits and constructs, and fucking uber gnolls everywhere. Stealth based characters are pretty much unplayable in it.

Neverwinter Nights vanilla had a terrible campaign. I had to force myself to complete the thing - I'd just put too much time into the game to not see the ending. Maybe I should have skipped the side quests. The biggest problem was repetition - the entire game used just a handful of terrain sets, and... It's difficult to explain how horrible it was. Every area looked exactly like the last one you visited with the same terrain set, and very little was done to spice up the surroundings - or maybe they tried and failed miserably. After trudging my way through dozens of sidequests involving the same generic-looking cave I was grossly disappointed to find out the final boss fight took place inside... yet another cave. I wanted to scream at this point. NWN is one of the ugliest games I've played, technically and aesthetically.
Another problem was the lack of rewards. About halfway through the game you stopped getting new stuff. All loot defaulted to +3 stuff. Everything. There might have been a few odd quest rewards here and there with an alignment or elemental damage modifier, but the thrill of exploring and finding shinies just evaporated completely after a while. From a character progression standpoint, that left levelling and roleplaying.
I can't actually recall if the roleplaying was good or not, but I assume it means it was a forgettable experience of soup kitchen versus orphanage slaughter with some arbitrary three-options-before-fight dialogue encounters. (Y'know, when you meet a friendly NPC, get three dialogue options which are basically love/fight/apathy and which all lead to a battle anyway and which have no effect on the game at all).
So, that leaves levelling. I played a Wizard, and if I recall correctly dinging took the longest to wear off in novelty. I mean, who doesn't like levelling up? Unfortunately, once you reached the upper tiers of spells, it was painfully obvious that all the "mechanics" had already been exhausted in the first four or five spell levels. High-level spells were either summoning spells (which were mostly very weak or easily dispelled) or instant death spells, or maybe the odd weak AoE. Reminds me a bit of the early Final Fantasy games, actually... Well, mostly I used metamagic feats to keep my repertoire up to date. Mostly Maximized Fireballs, until they stopped being viable, at least.
On the other hand, Shadows of Undrentide was a much more enjoyable experience. It didn't drag on nearly as much as the vanilla game - in fact, the 20 or so hours that could be squeezed from the campaign felt like it was just right. Oh, the plot was just a justification for smashing big things, you had to start at level one and it was a pretty linear experience, but on the whole it still felt much, much better than the vanilla game. Environments were more varied and felt a good deal more dynamic, thanks to all the little touches to give each area its own unique feeling; this made exploring feel more rewarding, despite the game's linearity. More varied item drops were also added to the game, to the point where itemizing became a fun progression path again.
Oh, and there was Deekin.
As for Hordes of the Underdark, well, I never really got into it, having other games on my mind (I got as far as the the area just past the lift), and a few days later my computer died. I recovered the save games (I think, might have to hook up my hard drive again), but why play NWN when I can play Unreal Engine 3-based games? :)
Ironically, despite being able to run Unreal Engine 3 games on full settings on my new rig, Neverwinter Nights 2 stutters even at mid-level settings. Gameplay-wise, the vanilla game is pretty much standard Obsidian fare: overambitious and falling a bit short on all accounts, especially the plot, where they tried to make it deep by squeezing in as many colourful characters as possible. It does fix most of the problems evident in the first game, although can the two really be compared?
Thank goodness for Mask of the Betrayer, however. This is the closest thing to Planescape: Torment I've played, plot-wise. Combat never feels unmotivated, story is absolutely superb with loads of replay value (hmm, I only ever played it once...) and the game feels like it's just the right length at about 20-25 hours. Unfortunately the spirit-eater mechanic kind of sucks - even though I was playing as a Paladin and only did the "nice" soul-eater thing (What was it again?), it still felt annoying that I had to remember to look at the meter as if it was the radiation meter from Fallout, despite the fact that it took ages for it to drop even one point.

I've been a gamer since 1989 and I've played a lot of RPGs since then.
I think you're being way too generous with the 'average' rating unless you're counting indies. Heck, make that even if you're counting indies. It was really awful for a big-name game from a reputed developer.

"Worst" might be a stretch, though I guess if I'm going to say it's not I should come up with an example of something worse. I agree that the NWN single player was pretty sub-par. The story was pretty drab, and the world doesn't have a living, breathing feel like, for example, Morrowind. It's more like a static set piece with obvious triggers for events, like "Step into this room and X happens, or talk to this person and Y happens.". It's a pretty good way to build a toolset that makes it easy for the community to build modules. But as a standalone experience I didn't particularly enjoy it.

*sighs* I'll give you that the OC campaign isn't spectacular. But there are many excellent modules out there that are great.
Morrowind's one of my favorite games. I dunno why you're saying it's completely lifeless. I agree it had a problem with generic dialogue for the minor NPCs, but there was some very nice and clever writing for the NPCs involved in the main quest.

Morrowind may not have been the best example. But compared to NWN, the world at least feels coherent, instead of having such an obvious trigger structure that it feels contrived.

some of these later replies calling the single player horrible is making me worried about my purchase. And for some reason the premium modules are choppy when I play them. Oh well.
Others have said the multiplayer is where the game is at , and I will try that one day. I just don''t feel like playing multiplayer yet with people who have been playing the game for years and years

I'd suggest trying not to let other opinions color your expectations too much and go into it with an open mind.
I've played it (yet to finish the OC as I get distracted easy) and while imho it's not as good as Baldur's Gate or Icewind Dale, it's still enjoyable. I particularly liked the town you come across where you have to act as a judge. I won't go into any more detail than that, but I really liked that quest.

Ooh, I remember that one. It was well-done and one of the few places where a choice had a major effect,

I am actually pretty sure I will find enjoyment from it. In my experience from gaming message boards (gaf for example), I usually am much more easily entertained by games compared to a lot of gamers. I try to have fun and not let the weak points ruin the game for me