I just find the ANH Vader looks a bit silly in certain areas. Like the helmet looks too big and the eyes are not fully black tinted.
I've always said that ESB defined the main characters... The movie provided the best incarnations of each visually, dialogue wise, story wise, acting wise ect.
I know ANH brought us into the SW universe, but ESB refined that universe. Look at images of both versions of Vader... It holds up better in ESB, he looks cleaner, meaner and the design has little to no flaws. The suit commands more presence as a result, and makes all of his scenes very memorable. I mean even the voice is better, the effects ect.