Bikers who take up an entire lane on the road when there's a sidewalk five feet away.
My #1 pet peeve.. 
^^^^ this was the very 1st thing i thought of.
i also hate.... gravy, ku, tuna, snakes, anti-americans, ku, dishes in the sink, bigfoot, kites, people that are late, bud light, the dude in the p&l bathrooms wanting a tip, aliens, crapty bartenders, and marmots.