SS2 gets easier when you get the laser rifle. I also recommend that ADAOB mod since the RPG mechanics are kind of unbalanced in the base game. also it's a lot harder if you're psionics, because psionics are sort of useless.
I never got into BG2. It's hard and it's hard to keep track of and you sort of have to be a bit familiar with AD&D rules to get the most out of it. I liked Planescape Torment a lot more once it got going, and I feel like Temple of Elemental Evil just has better combat mechanics than BG2.
I don't think NWN was very good.
I think my main problem in SS2 is I chose OSA. I know you are not locked in a class but I kind of used my first upgrades on their skills. Not that I even have any guns yet.
Playing BG1 so no idea on the quality of 2.
I haven't got that far in NWN but it seems decent so far. Only just left the academy though.