Super-duper-omega casual here, as in I only have like an 1-2 hours every few days to play. My play schedule would either be some ZM dailies/wqs, maybe some TW dungeons to get gear or level an alt a bit. Maybe do some transmog farming if I want to, but that's about it. Did manage to do an LFR wing at one point.
EDIT: Oh bugger! Misread your question... Yeah answer to your question, something like MoP or Legion I'd say, those were delightful in terms of casualness.