Flashpoints is for sure the most important. Its important for level50 to have something to without a to much big struggle to form a group, as well when leveling to be able to take a "break from quest" and do some flashpoints with a group
Operations can be skipped, cant see any reason there really. this ones can be handel trough chats @ fleet / guilds etc.
Heroic mission would come in second for sure
ppl can be able to wanna do them after they left the planet or if they chilling @ fleet etc. So that would be nice.
Myself only done a ew heroic2+ quests while leveling, so for future chars whould be cool to be able to do them all + all 4heroic