Words like 'library' and 'arbitrary' are pretty difficult for me. Luckily I don't speak English in my daily life.
What about February? That one's an awkward one.
The worst part is that if you actually pronounce it clearly and properly, it sounds wrong, because it sounds clunky, and nobody actually pronounces it like it's phonetically written. Most people just say something more like Febry or Febury.
Most people that I've encountered say "Febuary."
I do alternate between "Feb-roo-airy" and "Feb-ya-ree." It's not a matter of not being
able to pronounce it as much as being a slangy lad. Likewise I drop the T in "water," "rattle," "petty" half the time, and pronounce it the other half... I do veer wildly from precision to laze. I don't think I've found a word I
can't pronounce, though - not in English. "Ecureuil" is a bastard in French.
The weirdest one for me, in English, is "crisps." Which I suppose you don't have to worry about stateside, but it's one of those words that I can speed through really easily, but as soon as I start actively thinking about the pronunciation I get really caught up on the "-sps." It feels like there should be another vowel. Crisps is the kind of word someone would come up with to win a bet with a linguist over how many consonant sounds they can stuff into a single syllable.
I've got the R - W speech impediment going on slightly, so there are quite a few words I have trouble pronouncing. It was adorable when I was little but now it's just annoying.
See, this is where I always get very embarrassed, because it must seem like I'm massively taking the piss with my username. It must look like a sort of taunt. I'd offer to change it to wobwebwer, but I have the feeling that's just swapping to a completely different brand of offensive.