A fellow that we all knew by a wide variety of unflattering nicknames. His hygiene was a disaster, and was infamous for his huge scraggly and unkempt beard which had bits of the last dozen meals he'd had in it, and his infrequent at best baths made just sitting at the table with him a chore. When he brought food to the same table as you, he would leave a messy pile of dirty containers, half-eaten food & soiled napkins lying around. He fancied himself a master musician, and was fond of inflicting his poor attempts at music on his players at every opportunity, which usually meant he played Bards and wanted to use it as an excuse to sing loudly and off key (but thinking he's a virtuoso). Combine this with his general poor GM'ing (making sure every single NPC his PC's encounter is custom designed to negate all the PC's feats and abilities, or sending villains way too high in CR that kill half the party in the surprise round being one of his signature techniques), and he was known as one of the worst players and GM's my friends have ever known. His general poor social skills included going to others on his dorm floor at 2 AM and sitting down and chatting about inane nonsense when you're actively telling him to get out, that he's not welcome, and you have an exam in 6 hours and need your sleep.
At around the same time there was an almost equally odious fellow. He was fond of assuming he could join any game out there just by writing up a character and showing up, including if the game was at the house of someone he only vaguely knew. Now, having someone you barely know, and really don't like showing up at your door insisting on playing in your game because it's a D&D game and he's written up a character so you must let him in is really bad. What's the icing on the cake was that this was back in 2e era, and he always wrote up the most insane cheesy characters, with no ability score below 16, and frequently using the Complete Elves Handbook, having a set of Psionic Wild Talents that is about as likely for his typical Elven Fighter/Mage/Thief Spellfilcher as winning the lottery (remember those old random wild talent tables, I think he went through there and picked the best possible results). He also only played Lawful Evil PC's because he said that "all other alignments are too restrictive". At the very suggestion that even if you did let him into your game, he couldn't play an Evil PC or a Spellfilcher he'd throw a fit about how they were "Official" character options so DM's had to allow them into all games.
This guy even tried running a game once. It was like a train wreck, but not as interesting to look at. It was blatant railroading, but it didn't go anywhere nice, and it crashed after about 2 sessiosn when the PC's got tired of being spectators to a handful of home-brew uber NPC's (guess what their Race/Class/Alignment was?) getting to be show-offs and save the PC's from impossibly powerful creatures they could never defeat on their own.
Then there is another fellow, who while a basically nice guy, is really more a gamer groupie than an actual gamer. He likes to hang out with gamers, talk with gamers, buy and collect gaming books, but he hates to actually game. When he plays in an RPG he most often just sits alone in the background and, not interacting or roleplaying in any fashion. Instead he plays on his GameBoy or reads a magazine. He loses all interest in gaming once the game begins, but he loves to talk about it the other 6 & 3/4 days of the week.
Then there is the guy who can't roleplay to save his life. He's the purest incarnation of "roll player", who lectures other PC's on not being "Efficient" enough because they aren't taking his predetermined optimum character paths. If you were a 3.0 melee character who didn't have one level of Ranger and one level of Barbarian with the Power Lunge feat he thought you were worthy of no respect at all, and don't forget to Polymorph into a [insert monster here] every fight, and so on. He never roleplayed, and treated D&D like an exercise in accounting, where the biggest numbers win. I look back at a lot of this stupid exploits and rules-lawyery and see why the 3.5 changes were made, many of them were there just to plug the exact loopholes he was using. *shudder* He'd probably have his head explode at how the party in my current game gives about 75% of their wealth away as charity to temples and monasteries as they pass them.