In no order;
-Sogeking burning the world flag in Enies Lobby and Robin saying she wants to live
-Nami finally asking for Luffy's help against Arlong
-Zoro defeating Mr. 1 in Alabasta
-Luffy landing the first punch on Moria in Thriller Bark and Zoro taking Luffy's pain afterwards
-Luffy ringing the bell in Skypiea
Those are my favorite as far as momentous events go, there's others and so many nuances throughout the entire series that you could probably name one thing per chapter that was too awesome.