As the others have noted, generally BGP, as it defines AS (also see
https://en.wikipedia.org/wiki/Autonomous_system_(Internet)), but if you go by what makes for an AS, you could physically use other routing protocols, those other routing protocols design, though, don't support multiple ASs the same as does BGP.
In BGP, besides ASs themselves, you'll find other features that lends it to routing between different management of routing topologies, for example BGP's route aggregation, communities and routing policies.
That said, in the Enterprise sphere, if you route between different "entities", that truly define/maintain their own routing management, you might find redistribution between IGPs; effectively "AS peers".
