azerothcore is probably the best and most polished, if you don't mind wotlk
Tangent: Imho, the only reason it is good is because it's not as grindy and / or the community just didn't put as much emphasis on min-maxing things. GearScore was a thing of course, but theory crafting wasn't anywhere close to what we have now.
Played through pandaland, skipped warlords, came back for some of legion and then finally broke the habit.