Looks like Sakurai wasn't lying when he said he allowed the musicians to choose the songs they wanted.
List of new remixes per series in Ultimate.
Mega Man: 17
Castlevania: 13
Fire Emblem: 10
Zelda: 8
Pokemon: 8
Mario: 6
DK: 5
Animal Crossing: 4
Splatoon: 4
Metroid: 3
F-Zero: 3
Kirby: 2
EarthBound: 2
Pikmin: 2
Metal Gear: 2
Street Fighter: 2
Yoshi: 1
No surprise Mega Man and Castlevania would get the most remixes. Those two franchises basically raised the bar for music during the 8 bit era and I imagine many of the composers were probably inspired by one or both of these franchises and wanted to take there pick when given the opportunity.
Castlevania is particular is basically treated as music royalty. Not only are the new remixes incredible, the majority of the direct rips come from either the Judgement or Harmony of Despair version which amazing remixes in their own right as well. Of course the one negative I have is the version of Tragic Prince is from Harmony of Despair when I find the original from Symphony of the Night to be much better but I can see why Sakurai choose HOD version since it starts with more energy so it'd fit a fighting game better while the original was a slower build before it goes crazy. Other then that one nit pick, I couldn't be happier with the Castlevania selection and It's going to be hard not spending most of my fights on the Dracula's Castle stage.
Mega Man is right up there but the one negative is the majority of its music is from the Classic series. Only one remix is from X, and then we get a direct rip of X vs Zero from X5, Zero's theme from the first Megaman Zero and Shooting Star from one of the Star Force games. I mean the Classic series music is still awesome and many of the new remixes are top notch but it would have been nice to get a little more variety from all the different series like Castlevania got throughout its history. Still, at least the new remixes are top notch and really show why his original name is
Rockman after all.
Seriously, how in the hell is Top Man Stage this fucking awesome? I mean the original was nice and all but never one of my favorite Mega Man tracks, while this remixes just made it one.
Oh God the We're the Robots remix from MM9.
Goes for almost 3 minutes before it repeats. You know, I think that's one of the reasons why the composers are knocking it out of the park with this game is because Sakurai removed the rule that all remixes have to be around exactly 2 minutes that Brawl and Smash 4 had. Most of the remixes in Ultimate are around 2:30 minutes which is probably giving composers more room to go all out with what they want to do. For a music composer, having even an extra 30 seconds can certainly make a big difference when trying to develop a song.