For me, it would have to be:
TotalBiscuit - Because he's a very rude, uptight asshole to his fans, and he's even been rude to me personally on Reddit, Twitch and Twitter in the past, often without me actually doing anything to provoke him.
Pertemps - In short, they headhunted my recently updated CV and offered me the chance to go for a £16k salary customer service job, and expected me to either spend £90 on a train/bus commute to attend an interview between two night shifts which I couldn't reschedule, or withdraw my application.
Because I wanted this job, I went. Unsurprisingly, I didn't get the job because I was significantly sleep-deprived, fatigued from a three hour commute, inexperienced, ill-prepared and somewhat nervous, and had to compete with about 20 others who had years of customer support experience.
It wasn't even that which was why I won't do business with them now, but rather how callously they handled the rejection by phone and just rudely brushed me aside like I meant less than nothing to them.
Reed - I only use their website for finding work because it's genuinely the best around, but I would never use them as a recruitment agency given how much unwanted spam I actually receive from their recruiters. It's so bad, I've almost considered reporting them to a spam watchdog.