Among MLB teams, I'd say the best and most popular mascot is the Phillie Phanatic. Pretty easily too. Shout out to Orbit, the Astros mascot, but the Phanatic has been around for a lifetime and its routines are consistently fresh and hilarious. The Phillies are the gold standard of baseball mascots.
On the other side of the world, the Chiba Lotte Marines in Japan recently unveiled an amazing and also kind of creepy fish mascot at their games. To wit:
It's a giant fish that runs around the field and, for lack of a better term, barfs out its own spine so it can do a dance routine and other fun stuff.