The single step that takes me the most time is the thumbnailing. I'm a fairly slow thumbnailer, and I do the entire chapter in one go to make sure I get pacing and stuff right, and it can take a lot of time.
But in terms of individual pages... well, the ones with complex environments with tricky perspective are the ones that take the longest. They take time to sketch, they take time to ink, and they take time to colour and shade, because of the mass of extra details they add to the panels.