I don't think pure explorers have
General Order 24
If that exists of course (not some scam between Scotty and Kirk)
I think the main aim of the 5 year mission was exploration (even if it were between delivering medical supplies or errant colonists) however Kirk was always saying we're the only defense in this sector of space.
Was there any exploration in TNG though - any star mapping, any going out into the big black just for the heck of it? I can't remember that much but I may be mistaken.