It's Plant, since they're from a super well repped series and their moveset is fun but lacking in pizazz. Their trailer wasn't exactly hype inducing either, and it's extremely apparent they were supposed to be base roster. Fortunately, they were also given away for free for early adopters so it's like... yeah, okay. Overall, every new character has been fun though so it's not like anyone's "bad."
If we count specifically DLC I
paid for instead of getting for free, I
suppose Byleth. Since they're from a series that has a strongly established base in Smash and their trailer was pretty boring... but their actual moveset and stuff is great though, making them super fun to play and among my most used, up there with Terry. I've been wanting a weapon master style character for years and years and they delivered the concept well. They also, finally, finally, finally brought in a Fire Emblem stage that's based on a recognizable location from the series. Closest we had before was Arena Ferox and like... c'mon. That's not how that place worked in Awakening.
Fire Emblem has an outsized portion of the roster ngl. They are all good but Roy and Chrom could have definitely been replaced by characters from other series. FE takes up 8 character slots. That's more than Zelda and the same as Pokemon.
4 of those "slots" are pretty much just one character with tweaked variables. Roy and Chrom included. It's hard to be down on that when they're not exactly pulling as much dev time as a full character would.
