Tom Bihn TriStar would be the single choice. If I can combine another bag and still have it carry-on, then the Tom Bihn Co-pilot is excellent. Tristar for the bulkier stuff and clothes, Co-pilot for the things I want in the cabin (book, iPad mini, cables, etc).

This is my everyday travel choice, and I normally mount it all on one of those Go Travel luggage trolleys to give me the wheels if I need it. It all folds up for storing in the overhead, and I've never been asked to gate-check it.