In this episode of "Ask Hyojin", Hyojin explains what the most common way is to say "everyday". It depends on the person, but generally, 날마다 and 맨날 are more commonly used in spoken language and 매일 is more commonly used in written language or in more formal situations. There is no difference in the meaning among the three expressions, but if you are in a very formal setting, maybe 맨날 is a little too casual.

For your reference, in standard writing, "맨날" is written as "만날", but most people still say "맨날".