I dunno if this really counts but probably my most recent halo 5 play was in a custom game. I think the map was survive the mountains or something like that, it was just before the final part, I flipped my goose and ended up getting 10 or so headshots walking backwards down the hill with a recon dmr.
But probably my best play in any halo multiplayer was in ce on pc on death island 1v1 slayer, the score was something like 20-20 with 10 seconds left, the other guy had a shotgun, I had a sniper, my shield was down and I jumped back not realising their was a cliff, fired a shot and landed a headshot just as the game ended.