By impressive, I really mean IMPRESSIVE. I don't mean killing a grunt with the worst gun in any given Halo game. I'm talking driving a Warthog into a Chieftain and him throwing it with his hammer, but you surviving and killing the Brute in mid-air with a headshot from a Magnum.
My most impressive moment was in Halo 3: ODST, in which a large group of enemies was bearing down on me. I'd run out of rockets for my launcher, so I had to use the M6S SOCOM, The pistol you always start with. I had one grenade, and it was a Frag, so I leapt up, and punched the nearest Brute Captain in the face. The Engineer nearby was supplying them with shields, So I had to kill him first, or I would have a hard time. I couldn't, so I stood on him, initiating a Glitch I found out about while looking through the ODST glitches section. His tentacle randomly started flailing about, and threw a Brute incredibly far and killed him. Then I punched the Engineer and all Hell broke loose. Bits of bomb were flying all over the place, bouncing around then detonating. It killed them all. I lolled.
Do you have an impressive moment? It could be an awesome Warthog kill, assasinating the whole bridge crew in Long Night of Solace in Reach, Legendary, all skulls on, without being spotted, or maybe round 200 in Lone Wolf or Firefight.