I feel like I kind of have to give this one to Terry. I absolutely adored the Fighter's Pass as a whole and I might do a little review type situation later on for fun, but I think Terry was all around the best package I've seen for DLC in a fighting game period. First off, I can't understate the fact that he came with 50 songs that covered some amazing tracks throughout SNK's history. That's a huge value for people, especially when the other Challenger Packs were between 8 and 11 songs including Fire Emblem for some reason. Then, Terry's actual stage is one of the most impressive additions to Ultimate because it fundamentally changes how you play to a certain extent. It's like Shadow Moses Island, except way more fun because of how it fundamentally introduces a new mechanic to the game. And that's without even mentioning all the attention to detail put into the numerous SNK cameos on the stage that also correspond with different music selections.
And Terry himself is one of the deeper fighters in Smash history specifically because he has so much you can do. He really feels like the perfected version of the experiment that started with Ryu and Ken, and his moves are an absolute blast to use in the game. The super meter, while another comeback mechanic, leads to some amazingly fun moments too. He's just a blast to play and has plenty of little variation in how you can approach throwing out moves.
All around, Terry's addition is the closest that I've ever seen a single Challenger Pack come to feeling like a genuine expansion. It felt like it added a ton of neat stuff to the game and was a blast to play. It embodied the "premium DLC" idea that Ultimate seems to have adopted over Smash 4's more sort of uneven DLC offerings to a tee, and the only one that came close was Hero with his huge number of additions and the impressive nature of having 4 unique Heroes in one fighter. If the music hadn't been the sticking point for him, he might be here, but Terry just has to take the cake. Like Banjo & Kazooie is my personal favorite addition and I have a ton of fun with them, but it terms of analyzing a challenger pack, Terry wins by far.