Singleplayer, with a coop campaign on the side. I think everyone should do SP first/as their main playthrough, just because you get to explore and read at your own pace.
I've got about 50 hours played, Early Access included. Always end up playing way longer than I intended, game is addictive as fuck. And I can't wait to do another playthrough after I finish this one, because there are just so many different options and party members (I'm doing Lone Wolf with just Ifan as a companion, so tons of different companion stories to see.)
Probably the best RPG I've ever played. Just what you'd have wanted from a BG 2 spiritual successor.