I think you should go for those 3* character to level.
If I were you, I will go for Dr Doom as he can be useful for LR event and his ability does look scary once you have level on the blue covers which I have been missing it.
He can be use for most of the current event. Having Rag he’s only useful for LR event, PvE story he might not be able to use it as well.
Hope this give you a clear idea what you want for character to level up.