yeah, I think if you include the gif, McGregor x Aldo is probably the most watched fight ever, haha.
Most watched event for me is probably the PRIDE Final Conflict tourney, the last event from the tourney that featured Rampage, Liddell, Overeem etc.
Most memorable event or fight? Hmm.... Maybe the first or even second Chuck x Tito fight? I was young and 100% behind the UFC hype train at that point and just digested EVERYTHING they did.