It is hard to come up with some one greatest moment.. I've had so many
Good moments:
Getting survivor titles on my monk, ranger and warrior without trying hard.. just went naturally

Buying my kurzick 15k armor for my warrior back in the day that amber was really expensive

Getting my first green drop, rago's flame staff

Finishing Proph, Factions and Nightfall for the first time.. makes you feel uber..
Bad moments:
Having an error 7 after both the last mission in Factions and last mission in Nightfall.. the worst thing was that I didn't get my endgame greenie and was stuck with amulet and book of secrets again.. sigh.. fortunatly last missions are not that hard ^^
Having people ragequit cause they get killed while sprinting into battle when you monk and try to keep them alive as best as you can.. the insults you sometimes have to take when you try hard to heal, but the stupidity of some people still get's them killed..