I have basses with flats and rounds. Though I am a flats-whenever-possible guy, there are occasions I need rounds. FWIW, I only own 5 string basses...
I am still searching for my ideal flats. Chromes are good so far, though I am evaluating Cobalts at the moment. TI's are nice, but expensive and I have had issues with string balance. I love love love the sound and feel of the Sadowsky flats on the E-G strings. Their tapered B is not good for me and does not seem to match the rest of the set tonally. If you were looking for 4 string flats, Sadowsky's would be my first suggestion. They can get growly and angry but can still sit back and be a flat when needed.
For nickels, I use nothing Pat Wilkins Road Tested Nickels and I love 'em.
For stainless, it's the Dunlop Marcus Miller. They can be had cheap and they play and feel nice.
Note: all the above are light gauge, as that's what I likes...