Beating most any "hard" video game is always a great feeling just due to the sheer hours that go into it. In some cases, you have to develop the memory and skill to do the whole thing in one sitting. I can't count how many from the NES era fit this criteria. Top of that list are: Contra, Bionic Commando, and most Zelda and Mega Man games.

The best one happened in the middle of my Dark Souls play-through. I kept having to quit playing after short sessions, as skill and vigor checks kept wrecking me. This lead to anger and rage that just made it impossible to proceed. Once I made the connection that I could concentrate more and flow through combat more easily while calm, I changed tactics to calming my own mind and keeping it that way. The game just "opened up" after that. From there on, it was much more about meditation and breathing than equipment and leveling - skills I now carry with me everywhere. DS literally made me a calmer and more resilient person.