Anything goes well with Ana, because she’s Ana.
Zen is more about enemy team composition and who your DPS are. If they have their own Ana, heavily displacing Ultimates (Whole Hog), or burst Ultimates (Rip-Tire) then his effectiveness goes down with each obstacle. In cases like the ones I just listed, Lucio would be the one you’d want… Zen can still work, it’s just he has to work harder.
The best partners for Zen are everyone but Brigette and Lucio unless they’re pushing the Payload (Payload = 3rd healer). Or if there’s so much Defensive Allies or chip damage enemies that the bare minimum of healing can work.