I've tried a majority of the stainless steel strings made, always looking but at this point I've finally settled in on what I like. I'm older school, so I started out on Rotosound 66's, which was considered the classic SS sound. Bright, full of shimmering piano-like tones, with wonderful harmonics. They are a great string when they are new.
The problem is they go dead pretty fast, and sound like poo when they do die. Additionally, they are a little rough on the fingers. You build callouses, but when new and dry, they can hang a little on your fingers, slowing you down.
When I was touring, I needed strings evey now and then because I was getting some dead E strings fresh from the box. Somewhere along the way, I tried DiAddario Pro Steels, which I could find at any guitar center (nice when touring and knowing you can get something regardless of which state you are in) anyway, I really liked the tone. They had all the brilliance of the rotosounds, but they had a really nice growly tone in the midrange that stayed constant as the shimmer died a little. Where the rotos fell flat on their face after the shimmer wore off, the pro steels still had a great sound that seemed to last a lot longer before they died. So not only did they have all the brilliance, piano like tones and sensitive nuances the rotos had, they also had this killer midrange growl, were smoother on the fingers, lasted longer, and were cheaper.....
At this point I mostly use the Pro Steels for all my SS strung basses, as they just seem to my ears a better sounding string. The other stuff is just a perk. I will try a different set of strings every now and then, just in case there might be something better out there but so far nothing has been able to top the Pro Steels for my sonic needs.
I have started a tribute band as of late, and my pro steel strung P bass ran through an Ampeg SVT CL into an 810 cab sounds simply amazing. It's the rock sound I've always heard in my head.
I've never endorsed anything in my career, as I don't like to box myself in to a product, but DiAddario Pro Steels are something I would strongly think about endorsing if I ever decided to go that route. I am that sold on them.