4A: Blowing up the Spirit of Motherwill, AF Answerer, defeating White Glint or Stasis, the final mission of every ending, and being able to beat Occupation of Arteria Carpals. In order of awesome.
Beating Nine-Ball in Any Armored Core Game (including the Ultimate Nine-Ball in MOA and AC2:AA)
LR has its own moments, too, but the ones that stand out most are beating Uber!Zinaida and any of the Pulverizer battles. ESPECIALLY the lastone, who will rape you if you take on it unprepared.
Surprisingly, the opening cutscene of AC3: one of the black ACs gets its head blown off by the hulking heavy AC, who begins the slow and rather leisurely process of deploying its big guns to finish him off while the black AC stands, twitches, and sparks...only to rocket backwards and launch an missile barrage that would have put Macross to shame.
The final bosses of 4 and 5.
Verdict Day's final boss which is taking out a NEXT inside what is pretty much a Normal.
Also from Verdict Day, the Bonus Boss rematch with the Spirit of Motherwill, again because you're taking down an enemy that even a NEXT would have trouble with, with nothing but a squad of Normals.
Marche Au Supplice, Hard Mode, which pits you alone against Berlioz and his team in a four-on-one. It's by far one of the hardest missions in the game, but boy do you feel awesome when you finally manage to clear it. Same with the Occupation of Arteria Carpals mission in For Answer.