No feeling in an MMO compares to levelling your first character. When you’re not shooting for perfection. Not preparing for the next raid. When every little hop feels like a giant leap. And every mob you defeat – like a significant victory. When your quantum of achievement is a quest, rather than a quest-hub or a zone.
This is not to say nothing is equally good, or better. There’s great stuff down the road, many things to have fun with months and years later. I love being at level-cap and that stuff dubiously called “endgame”. Challenge and success, teamwork and friendship can be found there. Feelings I’d never rate lower or want to miss (as a matter of fact, right now, I am missing some of them, and it makes me sad). They are not worse or worth less – they just don’t compare.