Best 4*s to take to max-champ?

mpqr7

I realize this will probably take me years per 4*, due to vaulting. But I'm curious for those who have (whales and the like), what were the first 4*s you leveled up all the way to max champion level, for PVP and the like?

I am currently setting RHulk as my 4* BH. However my PVP opponents have included extremely high levels of the following 4*s: HBuster, Peggy, Iceman. Which ones would you do (or have you already done)?