Pretty relative question, IMO.
One answer is: the best B string in the biz is the B string in the hands of the best player in the biz.
Back to the instruments, we all understand scale length matters a lot for lower string projection. Upright players have struggled with this for literally centuries. Yes, plural. The 4/4 full size upright was made a few times; but pretty much everyone agreed you can't really play it. Some players soldier along with the 7/8 these days; and they do sound fantastic. But, they can be more work than fun. Most upright players use 3/4 with scale lengths around 42" +/- and call it good.
Back to electric bass guitars, same thing but different gigs. I'm sure some here are playing in styles where the low B response is a high priority. For traditional cover band situations, the B simply needs to be even with the other strings, rather than a long range howitzer. Most of the respected builders now produce pretty dependable B strings that will get the job done. If you really need the long range howitzer, then the answer is probably one of the 35" basses or the Dingwall; or, if you have unlimited funding, a custom long scale Alembic with ebony stringers and the Series electronics.
I don't need that kind of low B. In fact, very few cover tunes, even the newer ones really require notes below D, which I can actually get with better projection on the 4 with a HipShot. And, while the Modulus does have a thunderous B, I personally never really liked the way the rest of the bass handled. For me, the 35" scale is kind of like the 7/8 upright. Sounds great, but more pain than gain. I'll stick with my 34" B strings, thank you very much, from builders like Sadowsky, Zon, and Tobias (yeah, I know Mike has mostly gone to the 35"). That is what is "best" for me. In production axes, the SR5 has had a VERY respectable B for a long time now, and I have a gigged a Fender MM5 which also had an outstanding B. Of course, YMMV.