I've tried many types of flatwound strings and I have 3 suggestions depending upon your situation.
If you're just experimenting with flats, then D'Addario Chromes are a good choice. They sound good and feel good and they're not too expensive. The only downside to them is that the nickel plating wears off and the tone changes quite a bit, becoming very dull sounding.
If you mostly play on roundwound strings, then La Bella Tapewound is a good choice. They sound like flats, but they feel more springy like roundwounds.
Otherwise, go with Thomastik Infeld. These are wrapped in pure nickel, so there's no plating to wear off. They are 2-3 times more expensive than most other brands of guitar strings, but they last 10+ times as long. They mellow somewhat after a few weeks or months, but never become dull IME even after many years. For a super-mellow jazz tone, get the George Benson signature set.