For Double Bass or Bass Guitar? This is the DB forum, but the previous answer was about BG strings... Best steel DB string? For what purpose? Bowing? Jazz pizz only? Mix of bowing and pizz? Bright tone? Dark tone? Long sustain? Short thump?
Well, I don't have a true answer to your question. I mix and match strings from different brands, so I don't have a "best" steel string, at least as a whole set. I play jazz pizz only, and on an EUB (a Carruthers SUB-1), but I favor orchestral strings. They give me the warm, thumpier tone I like best. I like the Jargars for the top strings. For the bottom strings, I like Pirastro Original Flexocors, although I presently have a Pirastro Chromcor E installed. The Chromcor E has a better sustain than the Original Flexocor, and has a great dark growl on my EUB. If I had to use strings from a single set (no mix), I'd choose the Original Flexocors. (but the D, and particularly the G, sound much brighter than the Jargars) The Kolstein Varicor Excel is also an interesting orchestral string, but it sounds more nasal. Hope this helps!
So far, my only experience is with Thomastik Spirocores. I play mostly pizzicato jazz with the occasional arco passage here and there. While they sound GREAT for fingerstyle, my bass sounds like the Nanny when I bow with them. Much too nasal for my taste, which is to be expected I suppose.