Rankup all of them eventually.If you need power control champ - DoomIf you want damage dealer and end fights quickly - CGRColossus for immunities and tanky champ.Also consider what content you are tackling right now. All of them are useful.
Forgot to add option for Void.
Forgot to add option for Void. All of those options, including Void, are good options that you will eventually want to Max. Personally, I'd prioritize as:DoomColossusCGRVoidBut, all have their own particular strengths that will be useful for different aspects of gameplay.
Rankup all of them eventually.If you need power control champ - DoomIf you want damage dealer and end fights quickly - CGRColossus for immunities and tanky champ.Also consider what content you are tackling right now. All of them are useful. I am on the final quest of cavalier if that helps.