So, Mario has had 4 love interests throughout the series (well, 3 rather, I'm not sure if Rosalina ever actually was a love interest to him). So, the question must ultimately be asked; which one is the best?!?
Pauline: the OG, she is much less popular than the others, but she has seen a bit of a resurgence in popularity after her appearance in Super Mario Odyssey
Peach: By far the most popular, she has a very, very big problem with getting kidnapped. However, she is the one that is generally depicted as Mario's love interest
Daisy: while she is now more closely associated with Luigi, as a matter of fact her debut appearance in Super Mario Land actually had her as yet another love interest of Mario. For some reason she used to have darker skin that seems to have been toned down throughout the years.
Rosalina: The newest of the 4, however she has become pretty popular too via her appearances in Super Mario Galaxy, Super Smash Bros. and select Mario Sports games. I don't think she has ever been an official love interest of Mario though, which separates her from the other three.
So everybody, who would you choose? I'd say that Rosalina is mine.