At first I voted SPECIAL, because after the Underrail demo I'm replaying fallout and I'm having a blast.
But then I remembered that I also had a lot of fun with the AD&D implementation in BG and IWD and since it's going to be a party-based RPG, classes are better IMO. Class combinations make things more interesting, since each class has its specialties and its limitations. Classless character creation is great but mainly with single player games like Fallout and Arcanum. Yes, you could have followers, but it wasn't a real party. So, AD&D, or something similar. And heaven forbid, not something like the newer D&D versions, since IMO they translate badly into video games. I believe an AD&D-like system with even more flexibility (let's not forget its limitations weren't so good for Torment) would be the best option.
I hope there will be enough available party members to make varied party combinations.