Depends if Aizen underestimates his opponent, as he did with Ichigo. Aizen thought he was prepared to face Ichigo no matter what, and he wasn't.
Aizen's swords abilities are extremely powerful and if he uses them well he shouldn't really lose to any single character in bleach, though a combination would still be able to take him out.
Would he lose to people in other manga/anime, of course.
IMO Aizen solo's the Bleach Verse buuuut I haven't read bleach since Ichigo got a new Bankai. So I could be wrong. Outside of BV. There are many people who can beat Lord Aizen. (Aizen has been my fav bleach character since he fought Hitsugaya when he had glasses.)