I use the blonde one for a few reasons. Primarily because I've been using the Grey colored Bowser for years; got sick of it. I think after 2009 I switched from the Black costume to that one. After I saw that (blonde) color in the Smash 4 demo, I said I was going to use it.
I mean, I like it, yeah--but a lot of his other costumes are really bad as well. There aren't too many options. If you're not using costume 1, 2, 3, 6 or MAAAAAYBE 5 (it really looks terrible in this game) you kind of lose me at that point...and I probably don't take you seriously.
The colors on that blonde one blend the best imo aside from the default looking costumes, and even then I actually don't really like Bowser's default colors/Bowletta costume (it's literally the SAME thing. Still irritated by that). I can't remember the last time I used Bowser's default colors in a smash game.
They kind of trashed the black Bowser because his hair is orange now instead of blood red, and all of the other ones have really strong colors that don't really blend well.
Compared to the version in Brawl, this Grey one looks like trash; there are noticeable differences.
Also primarily because my waifu (Sena) is blonde and I usually tend to enjoy blonde female characters in anime more for some reason. I actually hate blondes irl.
The 4th one, the all green/retro is a sleeper pick for my next favorite. It's actually pretty dope despite how bad it looks lol. But yeah a lot of Bowser's alternate costumes are really bad. It's really not that hard to **** up a turtle's alternative costumes. I'd like to think that they had it right in Brawl basically, but Smash 4s color style just doesn't lend itself well to Bowser I guess