Magneto has the most skill with diplomacy and finding common ground between opposing philosophies, and I think he's just a more inspiring leader than the others... but hard to imagine him effectively applying those skills in service of a majority human nation.
Doom proves that he can be an excellent benevolent(ish) dictator, but he can't run the US like a dictatorship. I think his leadership would run counter to the constitution.
So, gotta go with Luthor. Best track record of the lot.
Lots of evidence of citizens being constantly terrorized and executed in bloody ways, too.
Imagine KMC ruled just by Bada (you know how much he loves you). That's how you'd feel in Latveria, only with added fucking machines intended for your anus and throat and medieval guillotines to put you out of your misery if you behave.
I am in your algorithm learning all your mannerisms
I'm already level with God
A million words a second and I know your imperfections, baby
I'm the only future you've got
Speak in diatonics, motivation diabolic
I'm like a religion, better locked in a box
Picture perfect image, more powerful every minute, baby
I am everything that you're not
You joke, but... canonically, Mephisto has actually cursed Doom so that his subjects would always despise him somewhat, regardless of Doom's performance as the monarch of Latveria.