Watching thousands upon thousands of Orks/Imperial Guards run into your well entrenched forces and DIE, DIE, DIE. Then, because you know it's a diversion, you send a few tankbusters to the one "Weak spot" in your defenses where his armored division comes around to be blown asunder. This is of course followed by your one skull probe flying into their base, succumbing to their stationary defenses...And sending a terminator squad, an assault terminator squad and two dreadnaughts next to their HQ. There's just no recovering his losses from that one.
Mainly though, just watching the russia-styled thousand man assault where it seems like your opponent believes that big enough piles of corpses will grant him a cover bonus and let him get a few meters closer to your base.
Needless to say, if nobody guessed it, Dawn of War is the game.