Profoto but not because they're especially good, they simply had the sizes I wanted, I could source them cheaply (even new they're mid price). They're also well supported with grids for each model and replacements should I need them. Ironically I'm replacing most of the Profoto softboxes I use with dedicated heads so the softbox I'll be using the most will be a Bowens Octo 120, when that wears out it'll be a sad day for me with Bowens no longer around.
One of the other reasons I sold my Rotalux softboxes was they're designed to be lightweight with thin materials, great if you're mobile but not so great if you want them to last a long time. Personally I think I'd be happy with almost any brand as long as I can source replacement parts, there's not too many brands that make a terrible softbox these days (some generic Chinese softboxes with bad materials do stand out though).