This one's a toughie.
Neither are major Mario characters, but at least Daisy has popped up in every single spinoff Mario game. (And was the damsel in SML1)
Geno was a one timer, as well, but he was a playable character.
Daisy is more widely known.
Geno was pretty popular in the Brawl days. I think he was among the top 5 characters.
To be honest, I prefer Geno, just for his obscurity, design, and moveset potential.
But an obscure Mario character getting in the roster kind of defeates the purpose of having an additional Mario character in the first place.
A big part of me prefers Geno partly because I always root for the underdog, and part for reasons I already mentioned. (Plus Megaman vs Geno would be awesome).
At least the Adventure Mode will feel a lot more epic with HIM around than Daisy. I used to support her.
Anyway... I'm going to be "selfish" and say Geno, because Mario fans who haven't played SMRPG should at least do so once in their lifetime.
Because I'd be doing a lot of those curious fans a favor by introducing them SMRPG in their lives.
One of the best SNES RPGS.