Planescape: Torment had the best plot but terrible combat.
Icewind Dale 2 had the best combat but a terrible plot.
Arcanum had the most freedom of choice but the worst pacing.
Neverwinter Nights had the best modding community but a terrible original campaign.
Baldur's Gate 2 is the only one that the best of all four without any of the downsides. Great plot, great combat, tons of freedom, and a modding community which ten years after its release continue to create new content!
Baldur's Gate 2 wins.
As for BG 1 vs. 2, think of it like Mass Effect. Sure you *can* skip to ME2 if you want, but it wont be nearly as good. Same with BG2. You can import your character from the first game into the second, so not only will your character be more capable than a default BG2 char
(In BG1 there are permanent stat boosts you can find for your character!) but there's also some equipment that carries over and you'll have a better grasp of the plot.
As for gameplay BG1 isn't quite as good as 2, but there are mods that improve it drastically. You can use the
BG1 Tutu mod to import the first game into the second games engine too, allowing you to play BG1 at higher resolutions with all the improvements the second game made to the gameplay. There's also the
BG1 NPC project mod that makes the dialogue and party interaction more like the second games.
With those two mods installed there's absolutely no reason
not to play BG1, unless you hate awesomeness that is, and the experience is so much more satisfying if you take one character from the very start of BG1 to the mind-blowing ending of BG2:Throne of Bhall too!
On a side note, this is the second topic today where I've ended up typing all this. Is there some sort of classic pc rpg renaissance going on I didn't know about?