I have two

In CS:Source the 3 best players at our LAN party were playing as Ts defending the position in cs_assault and we had like 8 CTs trying to win. It took us 6 hours before we managed to win for the first time. The map is so crazy unbalanced that we had to try so many different tricks, like hiding and sneaking in the shadows, timing our actions very precisely, entering at the same time, smokes, etc. So many strategies failed, until one actually worked. I still remember that feeling.

With my good friend we played (emulated) Doom II in a co-op sitting next to each other on a couch with laptops on our laps while drinking White Russians. Over the course of 2 days we finished the game. So many deaths!!! It was one of those times when if you looked away, the wall was shivering... I actually cooked Macbook Air on this. Some sensors melted around the CPU and it wouldn't be able to manage temp properly anymore.