I'm going to have to give a double nomination here. Surprisingly, neither of which is Mario Kart, though some of the boys can vouch when RPM was having a Mario Kart night and I vowed to take down someone we met on WiFi and when I did, he disconnected.
Double Nomination time:
-Gears of War: Although this wasn't so bad when I was playing Annex the night the mode launched (the people playing were awesome and full of sportsmanship, even the other team), the ranked matches are [tornado fang]ing retarded. Pretty much, the only "skill" is to know how to shotgun someone and god forbid if you didn't.
-Mega Man Star Force 2: (rubs hands) Oh, this is a good one. Usually, I couldn't care less about Auto-Tribe King users. They're pretty easy to dispatch and counter. My beef is with disconnectors. Yes, you're doing fine and taking me down gradually in HP, but god forbid I lock you in place and I drill your 2800 HP down to 200 in rapid succession. They immediately disconnect. Now, if it's a new player... 600 HP, mediocre card set, I'll happily accept it, since that's a really unfair pairing (I'm sporting around 2100-2300 HP...something like that). But when it's seemingly evenly matched and I take you down?
As a side note, most of these bastards are sporting a [TUS] tag in their name. Never knew The UnderSquare bred so many babies who can't stand being paralyzed in place, grass paneled, drilled by LeoKingdomGX and the DarknessHole'd.
I'd have mentioned Brawl for bad connections and people targeting me for playing Sonic, but I let my actions (and subsequent victories with the blue blur) speak nicely there.