There's such a range of different levels of grudge on this thread. I've namechanged as these might, all put together, be quite outing.
On the serious end, my brother-in-law is the most evil man I've ever met, and the world would be a far, far better place if he disappeared from it. Even better if he'd never been born.
On a lighter note:
The teacher who made me sit outside the headmaster's office when I was 5. I hadn't done anything wrong. My friend had told a lie (out of fear of the teacher I think) which got me in trouble. I had to sit there until I admitted it was me who had lied. I refused to do this (I hadn't) and spent the entire afternoon sitting there. Every teacher who walked past would ask why I was there and tut at me. Still seething. This would have been around 1985.
I hate a particular town in the UK because my husband's absolute cunt of an ex, a truly awful person who repeatedly tried to break us up and failed, comes from there. I therefore assume on some level that everyone from there is also a cunt. I dread having to go there sometime for work (not happened yet). It's probably a perfectly OK place.
When the AA 'demutualised' in the late 90s, everyone who was a member was meant to get a payout. I was incredibly poor at this time and very, very excited about getting a couple of hundred quid. They refused to pay it to me, because my membership had come as a package with driving lessons (a birthday present). WTF. Bastards.
Perhaps partly as a result, I gave up the lessons and have never bothered learning to drive since. I also won't have any product or service from Centrica, who was buying them (they also run British Gas hence they are also cunts for loads of other reasons).
I hate the lettings agency which, at around the same time, let a flat to us knowing that we were spending all the money we could scrape together to pay the deposit, knowing that my partner had cancer and needed to stay close to the hospital that was treating him, and lied to us that the owners had no plans to sell. Within days of us moving in it was BACK on the market - we found this out when we came home to see the FOR SALE sign up outside. They'd only rented it out as they hadn't been able to sell it. The lettings agency knew that, and knew we'd have been homeless if the buyers had wanted us out. We couldn't have got another deposit together at that time.