Archdevil
As the Demon Princes are to Tanar'ri and other demons, so too are Arch-Devils to Baatezu and other devils. These are the demigod-like rulers of the Nine Hells of Baator, or just plain Hell if you're coming from Pathfinder, the commanders of the diabolic legions and the other major contestants for Big Bad Evil Guys for players to defeat.
Because D&D's devils have a certain Abrahamic influence, there are traditionally far fewer of them than there are Demon Princes; in fact, if you don't count their boss Asmodeus as one of them, there's only ever eight Arch-Devils in total, plus a handful of former Arch-Devils who have the power but no longer have the authority. Who's who depends on which edition you're looking at.