There’s a case to be made for leveling being the most fun you’ll have in an MMO. You’re acquiring new abilities and new systems, exploring new areas, and usually getting something of a feel of growing power as you move on through various areas. Leveling doesn’t wildly change its format like endgame often does, either. That sounds more fun, doesn’t it?
Of course, at the same time endgame is when you get to play with your complete kit and can really have access to all of your character’s abilities working in concert. There are plenty of games wherein “being max level” or “having maxed skills” doesn’t mean that the gameplay radically changes, and you can focus on just playing the game you enjoy. If you’re playing Final Fantasy XIV or Guild Wars 2 or City of Heroes or several other titles, this can actually be more fun and still give you a reason to go back to older areas and content.
So there’s an argument to be made for both. Which do you personally tend to find more interesting, though, endgame or leveling?