I am surprised to see how civil this topic is currently, whereas I expected it to be unpleasant due to the nature of this topic. Here's hoping that this keeps up.
Favourite (assuming only one choice): Mario. There are a few characters I could consider favourites, but if there's one character that I like, it would be Mario because he's a mascot character that is organically one and not engineered as one (such as Sonic), since he has several characteristics that are not typical of mascots, such as being a human (typically animals are mascots), is not young (must be the moustache) and his design is based on design limitations to make him stand out. He's also a very cheerful person with the best voice because you will not expect a person like Mario to have a high-pitched voice that is also cheerful. Beyond this, the Mario characters I like are the ones that are outside the main cast besides Mario, which includes Daisy, Wario, Waluigi and Pauline, because they are featured less often so their appearance is more noteworthy compared to the likes of Luigi, Bowser, Peach and Yoshi (Toads are cool, though).
Least Favourite: Baby Wario. The only character I feel has the least amount of both purpose and charm that makes the character worthwhile, only because this character is made to be prominent (one of the Star Children!?) yet only appears in one game and never again. I guess money is important in Yoshi's Island DS so they made a character who does that and I guess getting 100% is easier with this skill. Even the other Baby characters appeared more often, and I even consider Baby Daisy having more of a point than Baby Wario mainly for appearing more often, despite starting out in a Mario Kart game!
Thank you for reading.