I'm going to do what I think of all the mercs, because I feel like it.
Scout because he has that 'childhood/college bully' theme going on, even to his teammates (Expiration date for example) but he does seem to try and care, but ends up still being a jerk to everyone when he does. But I wouldn't say I consider him the more unpleasant.
Soldier may be the most unpleasant, or close up on the list in my opinion. A lot of his comments are short and strict, he's racist to non-americans I think (dominating spy, medic, demo ect) and his overall moronic/dumbness causes a lot of people to be screwed over and/or killed. (Merasmus on many, MANY occasions, his own team in Expiration date, the old ladies and tom jones in ring of fired and many other occasions)
Pyro is no-where near unpolite. No offensive comments (obviously) and I doubt it even knows that it's killing people if Pyroland is accurate.
The Demoman can be unpolite in his comments, but he also seems to care for his teammates quite a bunch and isn't as unpolite as some, he's especially eager to help Miss Pauling and the gang when he's recruited in Ring of Fired, so he seems very happy to do what he does/help out. (even if it's just to get his mother off his back)
The Heavy seems to care for his team but of course, due to him not being so great at speaking English and being short-spoken quite often, and he doesn't insult the enemy team/same team often but he does seem to want the team to go away in A cold day in hell, but that's mostly because he cares more for his family then the team, until he's encourged by his family to go with the team anyway. He's polite to most it seems especially Medic, but he'd be unpolite if put between his family and the team in a situation that dangers his family.
The Engineer seems to be one of the most caring of the team, rarely getting into an anger with his own team and seems to try and be friends with most of them (Working alongside the Medic quite well in Expiration date), and he's rarely unpolite. He even says his domination lines in a not-so-angry tone with some politeness, so I'd say he's the most polite of the team. He also seems to help and support the individual (not saying the name to avoid spoilers) stay alive by giving them an australium life support machine + support to install it.
THe medic... Eh, I don't think he's polite much, if at all. He's got that basic medical niceness to him, but he just see's everyone as medical/test subjects and doesn't seem to really care as much for his team as anyone else. I think he's got a bit of 'Nazism' in him and seems to be willing to turn on/defeat his team (or earliest experiments as he calls them) quite quickly using the Team Fortress classic mercs (Latest triumphs in his words, all in A cold day in hell). I'd say he's the third most un-polite in my opinion.
The Sniper is 50/50 I'd say. He seems to respect his team and works alongside, greatly appreciating the help from the team to uncover the mystery of his birthparents but at the same time he doesn't seem as polite. He's very offensive in his dominating taunts, he drugs and ties up miss pauling and Demoman and he throws his own urine at people. I'd say behind the Medic, he's the second most un-polite merc.
The Spy is pretty impolite/offensive to his enemies but he has that sort of 'harsh caring attitude' to his team + Miss Pauling. I think he genuinely cares for his team, but in that harsh attitude in order to make them tougher and better... Except Scout, I'm pretty sure he loathes scout.
tl;dr: My opinion is that the Soldier is the most unpolite, followed by the Sniper and then the Medic.