For me it's restaurant. I'd say 99% of the time I fuck it up, and have to go look it up at dictionary.com. Then I'll work on memorizing it, even phonetically and 3 days later need to spell it again and bam! I mess it up again.
For some reason I just can't retain that word...LOL And the funny thing is I've always been a great speller. In 7th grade, they did some testing and I was reading and spelling at a college level however I sucked at math. But there are some words that just mess with my head.