You can't go wrong with a matching chair and ottoman. You can even have two disparate pieces upholstered in the same fabric to make them match. If you have a lot going on in a room, that continuity and cohesiveness of the fabric or material can really help create a more relaxed and polished look.
But don't get me wrong, I also love the look of a mismatched chair and ottoman — perhaps a leather chair with a woven ottoman. Having a mismatched chair and ottoman gives a room more of a collected, eclectic look.
Do you have a matching chair and ottoman?