I like flats but since I play exclusively 5 string now, it’s really hard to find a good B string in a flat
B strings can be a challenge overall, and I agree that it's especially true with flats. I've been impressed with TI Jazz Flats, and as noted the Sadowsky Blue Labels. LaBella Low Tension Flats had excellent tonal consistency across all strings, low B included -- the tone wasn't really my thing, but of course that's a subjective call, some really like them.