ĉefministro

Decoded: ĉef- plus ministro. The base is already on this site at rank 1,755, and ĉef- does the rest: chief, head, principal. Ĉefurbo is a capital city, ĉefredaktoro an editor-in-chief, ĉefstrato a high street, ĉefkuiristo a head chef. Learn the prefix once and a whole column of institutional vocabulary comes free.

Ĉef- is a prefixoid, not a prefix. This is the distinction the affixes as words article exists to draw. A true affix like mal- or -in- is still an affix even though it can also stand alone (male, ino); ĉef- was never an affix at all. Ĉefo is a perfectly good noun meaning a boss or chief, ĉefa is an adjective meaning main, and ĉefe is the adverb, chiefly. So ĉefministro is not a root wearing an affix, it is a compound of two roots where the first happens to be short, common and productive enough to feel like one. Esperanto has a handful of these - ĉef-, vic-, ĉirkaŭ-, sub- - and the practical consequence is that you may use them freely as roots when you need to, which you cannot do with -ig-.

The office, not the person. As with prezidanto and reĝo, the title takes no article and no capital when it stands as a role: ŝi estas ĉefministro, li fariĝis ĉefministro. A capital appears only in a name. And the deputy is vicĉefministro, which stacks the other prefixoid straight on top and stays perfectly readable, which is the whole argument for the system.