I liked D1 graphics (thought there was nothing much to love honestly, it was pretty much cathdral -> caves -> hell, and it all went for a realistic look in town.
I hated D2 graphics at first because they felt so...muddy. Even for the time, everything felt way too pixelated, but the environments were varied, which was great. The size and details of some of the buildings was also impressive.
D3 blew me away. From the non vertical environments, to the colors and lighting and vistas, I thought it was a fantastic mix of horrific and beautiful scenery, it displayed Sanctuary in more than just dark and grey emo appearance.
I hope they don't go the way of super edgy mode with D4, it would be a shame to D3's talented art team.