Best character in castle crashers

The four starters have mostly unique magic effects and are popularly considered the most useful of all 28 characters. An overview of some magic advantages: The blue is able to freeze enemies, while the red is able to continuously zap multiple enemies in his AoE splash move. The green is also popular because he can poison enemies and continually do damage without attacking further. The orange knight deals fire damage but has no real advantage over anyone else. The demons that you can unlock by beating the game with him have very similar special moves.
Of the four starters, the red is the most popular and probably the most practical to use. But there are some very great secondary characters. The Alien from Alien Hominid can cast at a 3x faster rate than any other character. The bear and the Saracen can both morph into tornado form for up to a minute when their magic is maxed out. The industrialist and fencer have an amazing splash damage spell which is the bane of bosses everywhere and an easy way to level up quickly. The king can heal himself and others.
In short, there is no "best character". It depends on you.