The most irritated I've ever been was when I took my first Factions character (my ele) to Tahnnakai Temple. I just couldn't beat it! I left it for months before coming back to finish it off. To this day I still don't know what I was doing wrong, because my mesmer breezed through it!
The most fun I've had lately was when I got
Then and Now, Here and There and suddenly I was back in the Diessa Lowlands for the first time in a long, long time. The whole quest made me smile.