Which Is Correct Ruder or More Rude?

The word "ruder" is just fine, and probably more frequently used. What sounds better will depend on context. He is more rude than anyone I ever met.

Is a ruder a word?

1. insulting or uncivil; discourteous; impolite: he was rude about her hairstyle.

Is rudest a real word?

Superlative form of rude: most rude.