My first 100 is maybe the best moment in my video game history.
My friend and I each had a copy of Fire Red and Leaf Green, we had started over together and been training for an ultimate showdown. We were in high school at the time and with winter break on its way we decided we would battle at the end of winter break.
Winter break reaches its end and the battle begins. The levels of our pokes are scattered between 70-80 and its a close fight. Enter Jolteon, level 100. Game over. The look on his face was priceless, I wish I could have a moment as amazing as that again with pokemon.