Just going to say this. With the mechanics of this game being as they are, Bowser has benefited greatly to the point of being a real threat. While I did feel that Smash 4 Bowser was also very good, this is definitely the best version of Bowser to date. Bigger, better, faster, stronger in virtually every category. Hell, even his fire breath does near brawl-levels of damage. 25-40% of free (and mostly safe) damage is a major boon in getting damage up to killing levels quickly and efficiently.
Now, this is not to say he's extremely amazing, but after this past week, I can confidently claim that Bowser is very, very strong. I still love Mega Man, and I absolutely am in love with KKR and Ridley, but Bowser is still the one true King for me.
The hardest matches I have come across have been against Marth/Lucina, Pikachu/Pichu, Ness and the Belmonts.