I unenjoy most of the same things the rest of you listed. My addition is more about a specific behavior than a player archetype. For context, this is a dealer's choice game with a lot of double-board and other split-pot games, so showdowns can be tricky.
A regular player with a lot of other annoying habits has recently been dragging out showdowns like this. It makes me crazy.
Even after everyone else has tabled their hands, he sits there staring at his cards with his mouth half-open, trying to figure out what he has. The later into the night (i.e., more beers) and the more complex the game, the longer he sits there staring. Half the other players are telling him to just show his hand, and most of the time he keeps sitting there staring at his cards, prolonging showndown quite a bit.
Even that is only moderately annoying. It gets worse.
Suppose we get to a point where I ask if he's conceding—y'know, so we can move on with our damn lives—and suppose he says yes. I used to take this opportunity to chop up the pot, award it to the winners, and begin clearing the cards, as one does. Now I know better.
Even after he says he concedes, he often wants to hold onto his cards and keep staring at them. A few times, as I was clearing the board, he blurted out, "Wait, hold on, I think I have a flush!" or whatever. This is way past the point where it's appropriate to try to contest, as the pot has been awarded and the other players' hands (and sometimes the board cards) have been collected. He even did it one time after throwing his cards face-down. After I've already awarded the pot, he reached out, grabbed his mucked cards, and tried to claim he had a winner.
There's no way I'm awarding him anything at this point; that verbal concession is a done deal. But sometimes he gets irritable about it and acts like he's been done wrong because he voluntarily forfeited half the pot without showing his cards. Now, because of this, I won't accept a verbal concession from him. He needs to table his hand, or it needs to go irretrievably into the muck, and until that happens I'm not moving anything. Anyone else can verbally concede, just not him.
It's such a ridiculous babysitting task. If you want your hand protected, table it. Then we have to award it if you have the winner. It's so simple.