Canon, Nikon and Olympus all have very good reputations.
Canon has an insane lens range, supplemented by a ton of really good third-party lenses.
Nikon has nearly as large a lens range, supplemented by a ton of really good third-party lenses.
Olympus went with a ground-up redesign, and the "Zuiko" lens range is getting great reviews. It's not as broad as either Canon or Nikon (but not all of those lenses work well for digital APS-C photography), and the E-series doesn't have as many third-party manufacturers yet.