Greatest victory was finishing Medal of Honor: Heroes 2 for PSP. That very last part was **** hard for me. Nazis keep coming and the facility was going to blow up. Half of the 42 times I died was from Nazis killing me and the other half was the facility blowing up.
Recently my greatest win had to be what ChillzMaster ahs said too - beating Halo: Reach on Legendary alone. It was one of the toughest things that I had ever had to do in a game and it's one of my most treasured ones too!
Oh, as for flash games I think that it would definately be beating Raze 2 with one second left! Pure Luck!
Newest victory: Solo kill Diablo in Diablo 3 in Hell difficulty with ONE POINT OF HEALTH LEFT on my level 60 barbarian. I'm still shocked I was able to solo him, since i refuse to use the auction house until they get it to a stable condition.
"A Monument To All Your Sins" (Beating the game on Legendary, ALONE).
Bingo. That took me at least 20 hours of gameplay. A recent one of mine was finding a single block of coal on the first night in Minecraft right as the sun was setting.