Thanos, easily.
His selection of me, would mean that I've reached a status that can take on, and beat heroes. This also means that I'm his personal guard, which he keeps around all the time. Plainly put;
It took Peter Parker and Doctor Strange nearly dying, to beat Ebony Maw
It took a Captain America as a distraction to kill Corvus Glaive....and that's after he wanted all the smoke from Vision TWICE....both of which he won.
Cull tanked Iron Man and Parker..EASILY, and he was the weakest of the order.
Proxima Midnight, 1 v 1 Wanda....won twice, they needed to 1 v 3 her to even make it fair.
Working for Thanos would mean you'd have to come with all and every parts of the smoke to even challenge me.