Other whales, sharks (including great whites), rays, seals, sea lions, otters, porpoises, dolphins, fish... depending on the pod, where they're at, what's available, transient vs resident pod. They kill pretty much everything except people.
They have been known to try (Scott, in Antarctica, I think?) and who knows? Orca poop don't talk.
We watched a couple orcas take out a grey whale off Point Hope, from the boat, about 50 yards away. Felt kinda sorry for the grey, but that's the way Nature rolls. We didn't feel the need to dive in and try to rescue it.....
. A couple times we saw greys right up on the beach, partly out of the water, and Orcas or Orca farther out.
IIRC, they have even taken swimming deer and maybe moose in SE Alaska.