AT.
The IBM PC lineup started with the PC (5150 and variants if any), then the PC/XT (5160 and variants), then the PC/AT (5170 and variants).
The PC and PC/XT used one particular keyboard interface, a 5-pin DIN plug with a particular pinout and particular protocol. The PC/AT introduced a new interface using the same physical plug and same pinout, but with changes to the protocol that prevent the keyboards being interchangeable unless they're the switchable type or autosensing.
PS/2 uses the same protocol AT did, so a simple adapter with no conversion circuitry allows AT keyboards to work on PS/2 ports and vice versa.