D2 Remastered. I have zero faith that D4 will turn out any better than D3, which I was massively disappointed by. D3 plays well, I'll give it that, but it completely lacks the oppressive atmosphere and art design of the first two, and that's the entire reason I played those games. Hell, I didn't even like dungeon crawlers in general when I played Diablo 1 as a wee lad, and yet I stuck through till the very end just because I couldn't wait to see what creepy, weird, mildly terrifying room I opened next, and every play through was different due to the randomized map so it still feels fresh to get into even today. The only thing about D1 that's aged badly is honestly the controls, so my happy medium would be a D1 and D2 remastered package that comes with improved controls and a run for D1 ala the Hellfire expansion.
I wouldn't mind a D4 if the artists, writers, and environment designers of D3 were kept far away from it and they instead focused on the atmosphere of D1 and 2 as something of a baseline for which to build off of.