(Obligatory)
To crush your enemies, to see their mechs driven before you, and to hear the lamentations of their commanders.
(More serious)
Running into a tough mission, surprised by enemy forces, or simply outgunned, and managing to pull through to victory with superior tactics.
One of my favorite memories is being out flanked on a destroy convoy mission. I had a perfect intercept position in a mountain pass. Enemies roll up with a Demolisher, then a reinforcement Jagermech starts shooting my mechs in the back from beyond sensor range. Juggling range from AC/20s while not exposing myself to the sniper was a lot of fun.