The Scout, barely beating the Medic and the Soldier.
He is a self-absorbed, violent little piece of shit. You can probably blame his childhood for a lot of that and Expiration Date showed that he has the ability to unterstand he is not the only one capable of feelings... but damn. He's that cool highschool kid gang leader, always on the lookout for someone he can humiliate to subdue that feeling of abandonment and loneliness his fatherless upbringing has left in him.
The Medic is... not necessarily evil. But then again he is definitly not your classical nice guy. He is polite and the youthful joy he gains from his work is refreshing and a pleasure to behold. But he will not hesistate to sacrifice anything or anyone in the name of his scientific progress. He will keep you around and care for you as long as you are part of one of his precious experiments, but as soon as something more interesting appears you are forgotten.
The Soldier on the other hand is just unpredictable, violent, mentally instable, self-centered and has little to no unterstanding of the social world- or generally the world. Maybe Zanna will help him become a reliable murderous psychopath, but I doubt it.
Concerning the others: I leave Sniper alone, he leaves me alone. He's a professional (most of the time) killer, but he seems to be able to take some time off and be a normal human being.
Pyro is like a pet baby dragon: Cute and dorky, but you mustn't forget that he is capable and maybe willing to burn everything you love, including yourself.
Engineer: I like the man. He's an intellectual, he has at least some unterstanding of art, he's polite and knowledgable while keeping his cool and being a pretty down-to-earth guy regarding speech and manners.
Regarding the Spy: I'd agree wih /u/youbutsu on him. He is bitter, no doubt about that, he's quickly angered and yet never really expresses his feelings so much that you might guess what he really thinks and feels. But honestly, how can you watch Expiration Date and not see just how noble, generous and caring of a man he can be? I'm pretty sure, had he picked another profession, lived another life... oh well. Too late for that now. It's tragic to know that this potential has been mutilated by years of deception, lies and loneliness.
Finally the two guys I consider to be the most pleasant company: The Heavy and the Demoman, two very different people but both in their ways pretty likeable.
Heavy isn't happy or cheerful. His life was much to grim for that. He is calm and lives very consciously, savouring every moment of silent joy, every moment that is not people shooting each other. He loves his family and in their absence takes care of his gun instead, sends them money and personally protects them when his source of income is gone. And despite his somewhat broken English he is a smart man, serious and composed.
Demo is the opposite, a loud and cheerful guy who is pretty much your stereotypical working-class man down the street. Loves sports and hitting the pub with his pals and he not only takes pride in his work but also really cares for the other mercs. Not as co-workers, but on an emotional level. That merc job is his life and the mercs are his comrades on the crazy rollercoaster ride that is working for Mann CO. He isn't blind for their flaws and I think he would like them to appreciate each other more (which is why he didn't exactly care about finding Sniper)- but all in all I like him for his almost innocent love of explosions and generally loud, stupid things. He would make a great Mythbuster.
tl;dr: Scout is an annoying brat, Soldier and Medic a completely crazy, Demo and Heavy are cool guys.