I think it's pretty inarguable that Crash and Geno are the top dogs of Smash speculation right now. They're two naturally popular characters with a lot of evidence perceived in their corners. Those two factors are, historically, the biggest predictors of popularity for Smash. People like betting for winners and people like betting for... well, people they like.
It's hard to find a third who fits the bill as well as two previous two do and, thus, can match their popularity. a Gen 8 Pokemon and Rex are two characters thought to be very likely, but they aren't super popular in a vacuum. Likewise, characters like Doom Marine, Sora, and Issac are highly requested but each have a roadblock to their inclusion that stifles support. Even characters like Bandana Dee and Dante, two characters who are naturally popular and many think have a good shot, have enough debate on their exact chances to keep them from reaching Crash and Geno's level.