I couldn't possibly pick an absolute favourite, I've overused sandstone and stone bricks over the years which makes me more likely to oick them because I'm used to them. Recently though blackstone has shot up and very quickly become one of my favourites, definitely try to use that where I can in new builds. Red nether brick I've always liked, but could never get it in large enough quantities to use it as much as I'd like.
Also, potentially controversial opinion: regular brick blocks look amazing after the re-textures, if used right they can look great. Maybe not my #1, but it's up there. The same goes for end stone bricks, they're hard to get right, but can look good if done well.
Oh, and grass blocks, but only in jungles or roofed forests.
