I use Ernie Ball flats on one of my P-basses and I like them a lot. I use the light guage (40-95, I think) and, to me they are right in the middle of the flat spectrum in terms of tone and tension.
Tone-wise, they're not as deep as LaBellas and not as bright as Chromes.
Tension-wise, they're not as loose as TIs and not as tight as Chromes or LaBellas.
For me, they're a really good all-rounder, but they don't seem to get a lot of love here on TB.
My other three basses sporting flats:
Danelectro - LaBella Danelectro flats - nice and thumpy, super-light guage, so not tight.
J-bass - RotoSound 77s - not your typical flats; lots of mids, medium-high tension, not as smooth as some others. They sound great on the Jazz.
'51 P-bass reissue - TI Jazz Flats - Love 'em. Low tension, tons of mids and detail.