In addition to Baldur’s Gate, you might also want to try out Neverwinter Nights. Neverwinter Nights is based on 3rd edition DnD, unlike the earlier Infinity Engine games which are based on 2nd Edition. I played it a long time ago and have mostly fond memories of it, but honestly don’t know how well it holds up. The first Dragon Age: Origins feels like a continuation of these games (and yes I do believe it is actually pretty good to answer 2501 ), although it is definitely simpler than say the Baldur’s gate games. Unlike those prior games it uses it’s own DnD inspired system, but it feels very much in line with the way DnD systems were implemented in the earlier games. They all utilize the real-time combat with pause system, which takes some getting used to. It can be fun, but personally I greatly prefer turn-based. There is actually one game which reproduces DnD 3.5th edition in a tactical turn-based engine (which is of course closer to the actual tabletop system), namely Temple of Elemental Evil which is a fun dungeon crawl, but light on story. For a modern game which scratches some of the same itches as the early Bioware games, but with turn-based tactical combat, I think Divinity 2: Original Sin is very good. Combat is much more engaging than real-time with pause in my opinion. I am looking forward to Baldur’s gate 3 as the same developers will implement a turn-based version of 5th edition DnD, which is the edition I have played the most in real-life, but who knows when I will actually get around to playing it.
One thing I will note about a lot of these games is that I have often played less than half of them before stopping for whatever reason. I still really liked the often relatively substantial number of hours that I played, however, so I don’t necessarily consider it a mark against them which is why I still recommend them. I haven’t played it myself, but I have a friend who very enthusiastically has tried to convince me to play Arcanum, seems like an interesting game as well.
Planescape: Torment is my favorite out of all these, but the combat is not super interesting. However, I do believe that the narrative design is somewhat intertwined with the game-play systems, more so than one might think when people recommend it on the basis of the story alone. The game is based strongly around dialogue choices and these are the best ways to level up you characters as well. This does mean that stats like intelligence and wisdom are overwhelmingly the best choice for the initial allocation of stats as it opens up more dialoge choices which allows you to experience more of the narrative. Sigil is exactly what I want in terms of exploring a world, i.e. not an open world, but a small dense area that one gets to know well, with interesting characters and dialogues everywhere one turns. The central story and mystery is also very satisfying with some nice writing in general. I feel like some fans of the game tend to oversell the writing somewhat, although it is better than in most games. However, it also mostly works because it is a game where one interacts with characters in an environment with choices. I think it would be difficult to take the story and fragments of the writing and turn it into a fantasy novel that is anywhere near as compelling as it is when you actually play through it, for example. This is a game that I actually finished!
As you mentioned Diablo II, I think that Titan Quest and Grim Dawn are both good games in the same mold, the main attraction being that one can choose two classes and mix and match skills between them. In general I think that Diablo-like action RPGs are a completely different genre than the type of CRPGs mentioned above, however.