I got a couple here, so bare with me.
FF7 - When I, through a miracle of god, beat Emerald Weapon. And without the Final Attack Materia too. It took almost 20 minutes, and constant casting of Knights of the Round and Mime. Towards the end Emerald used that doomsday attack, whatever the name of it was. I thought for sure I was dead, 2 of my guys bit it, but some how it missed my 3rd guy, who happened to be the only one with the pheonix summon. It was a 2 year obsession for me and that game. Since the moment when I beat Emerald I haven't touched it since.
Wing Commander III - When I tracked down and killed that damn traderous spy HOBBES. Just as a side note, for those that played that game, Ever notice how the Minmatar Typhoon bares a striking resemblance to the Wing Commander III Beheamoth Planet Killer?
StarLancer - When I finally was able to kill the Sabre attack fighters, I think thats what they were called, before they could destroy those crab lookin ships that set the charges on the Station at the end.
Combat Flight Simulator - When I finally mastered the fine art manual fuel mix on the P-51D and I blew my flight instructor and mentor out of the sky. This was an online WWII flight sim for those who don't know.
Star Trek: Borg - Best fully Ineractive Movie Ever! Beat it all on my own. No online walkthroughs
World War II Online - This was during one of their more stable patches. Myself and 3 squad buddies snuck behind enemy lines and set up Flak 30's just outside of an active Allied Airbase. Shot down Blenheim Mk1's and IV's for 2 hours before we got strafed by Spitfires.
I'm sure there are more, but I'm too tired to think of them.