Acid strength is based on the tendency of an acid to dissociate into a hydrogen ion and an anion. A strong acids are able to completely dissociate while weak acids can't. The easier it is for an acid to dissociate, the more acidic it will be.
So on the periodic table, the least electronegative element is iodine. Thus the strongest acid will be HI which is hydrogen iodide
Base strength is also the tendency of a base to separate into a hydroxyl ion and a cation. Strong bases can do this completely and weak bases cannot.
As with acids, the strongest base will be a combination of the hydroxyl ion and the least electropositive element. And it was LiO which is lithium mono oxide. But recent studies and research created something called a superbase.
The super base is called Ortho-diethynylbenzene dianion. It was synthesized in Australia and has taken the place of LiO as the strongest base