Andrew Ryan wasn't really a boss, as you technically don't fight him. But by far better than Alduin.
That's true, but are we talking about bosses as in boss battles? Or bosses just by going by the game's Big Bad? If the former, than I'd have to remove Joker from the list as well, but both of them are much more interesting characters than Alduin is.
The OP doesn't state anything about boss battles, which makes me look at this as more about the game's "boss" level antagonists, regardless of whether or not they are people you physically fight. And one can even make the argument that considering Ryan pretty much embodies Rapture and its decline, the entire time you're fighting you're fighting him.
The OP mentions specifically writing and visual design, not the final battle. Going purely off of writing, Ryan blows Aludin away, even with what I feel to be his http://www.youtube.com/watch?v=f18NWnxRmjI being removed from the game. Visual design doesn't really come into play on this one because it's comparing apples and oranges. One looks like a man, the other a dragon, and with the difference in tone and visual designs, it's hard to really compare the two games on that level.