Whaling on amateurs is fun, but I hate it when I end up facing players skilled with someone I rarely fight, so I have no clue how to fight effectively, and usually lose, or win by the skin of my teeth.
I also hate extremely luckies, and KO meat puppets. Like a game with Ike and two Foxes who happily skip right into his smashes, or a Bowser no one can seem to KO. Then there's the games with a genuine skilled player, but the others are too dumb to actually acknowledge this, so they either throw themselves into his/her smashes, or completely ignore him and focus on eahc other...and me. It gets annoying when I'm facing a worthy Falco player, and I keep getting knocked out of a dodge-roll by another player, and right into his smashes...