So. Many. Opinions! And here's yet another...
You'll find absolutely no love whatsoever in my house for any LaBella or TI strings. The former are pure sonic mud (I've tried DTF and LTF, *and* a set of their SS rounds. Barf!), the latter are like little elastic bands, and I just cannot take them seriously. I realize these are by far the two most popular things among the hive mind here, but that's been my experience. My P-Bass is something of a "treble eater" though, and so I find it gets a more balanced tone with slightly brighter strings.
My favourite for a while was the GHS Precision flats, but after using stainless steels on that P for many a year, I've got some Roto 77 flats on it now. These are great (for flats)! This is my first experience with them, but no problems whatsoever with balanced output that others have mentioned here. They are flats, so it's definitely a warm, old school thump, but there's a little presence and zing to them which I rather enjoy. I only use this bass now for old school soul/RnB gigs, so it doesn't need to do anything but be thumpy; hence my return to flats.