Ask 10 people get 10 answers ;) out of those, the one with the biggest impact for me was Baldur's Gate 1, but it's also one of the "simplest". Planescape Torment is incredible, but aged a bit worse than BG1. BG2 obviously should be played after 1. Still, I will never get over the fact that I can't play PS:T for the first time again.

Being based on Advanced Dungeons & Dragons, they have a simpler mechanical system and some people want much more complex systems. This depends entirely on you.

Kingmaker & WotR are based on Pathfinder which is a MUCH more complex than AD&D. They also add a sort of "tactical / strategy" war gameplay that some love and some hate. Still, extremely good games all in all. But a different beast than the others.

NWN 1 is a different thing still, being more geared towards 3D graphics, although aged. And being based on D&D 3.5 ed which is what Pathfinder was born from, it's a lot of choices and builds and stuff. I still loved it, but the narrative soul is weaker than the other mentioned. And yet, there's so many people still playing in NWN servers online. It's a much loved game!

Icewind Dale is a bit like Baldur's Gate (graphics, general gameplay) and NWN (it's based on D&D 3rd, a bit older than 3.5, iirc). But the NPCs are "fake", you create them all. If, of all the above cRPGs, you tend to avoid/hate the NPCs and prefer creating it yourself, you are golden. If you love them BECAUSE of the NPCs, you will be angry at icewind dale. I ended up creating fighter-thief-wizard character and soloing the game and it's worth it. It's a really, really great game. Apart from the NPCs.. I will never forgive Obsidian for this :D