Hmm... the most fun event? I honestly can't think of any. Odd. I could swear we had some fun events... I mean, Doki Doki was interesting... the dancing was fun... the being kept to an island all the time that was overfilled causing massive fps and lag... not so much. The Festival going on right now? Suppose it's sort of fun... I mean it has minigames you can do, not that I've seen anyone do them.... I used to enjoy those pet dungeons, they were great fun... also a good ground for meeting new friends
Well we all love those, but are they fu- yeah I suppose it is fun repairing at Ferghus' with no chance of failure
I think the most fun event I ever had, was a guild hosted event where we would bring items to certain guild members and then track down one certain giant called Santa in Vales. That was a blast! And so was the oversized party dungeon runs we did... then again, they were a tad more difficult back in the day!
It's quite sad that I can't really think of a super fun official mabinogi event though...