What is correct, “thanks to all”, “thanks all” or “thank you everyone”?

ANS:

They’re correct in different situations.

Thanks to all who attended our event! (You typically need a “who” after this one, so you get “thanks to all who [did x]”)

Thank you, everyone, for your support in this difficult time. (This one sounds the most earnest of the lot. You can just say “Thank you, everyone” — I added to it to convey the sincerity.)

Thanks all! ^_^ (The most casual — it’s not grammatical, but it’s good in intimate settings. You see it on emails and such.)

Another good one:

We thank you all for coming. (Or just “thank you all.” This is probably the most common.)