Team Rocket is an obvious choice for this thread, simply because they use their Pokemon for the purpose of evil and Pokemon are for the most part not inherently evil.
Even though someone already said similar in a post above...
I disagree. Jessie and James seem to take good care of their Pokemon & care about them. Jessie didn't always use her Pokemon for evil, either. She entered contests and showcases with them, too, don't forget that. So, they don't only use Pokemon for nefarious purposes. Just...most of the time, they do. But, I guess using them for bad puts them up on the worst, I don't think they're the absolute worst trainers.
I agree with Tepig's trainer wholeheartedly. Leaving Tepig tied up like that was horrible, and he didn't deserve the Pokemon to begin with if he was gonna do such a thing. =/ Daimon was just really shallow, honestly. I think this is a pattern for every Fire Starter Ash is going to capture. By the way, while not the worst, Shô wasn't that nice of a guy, either.
Most unfriendly? Easily both Paul and Trip, to some degree. Paul especially, since he has a reason to be an ass. I liked that about him, and it's why he was the better rival than Gary ever could be. Trip is just a stuck up snob, and it was pretty aggravating imo, but he wasn't that nice either, lol. Man, Paul was good, though.
Iris isn't friendly either, but nevermind her.