I'm a pretty good speller, but the most challenging words would have to be phobias. (Here I go again with the phobias... )

Hexakosioihexekontahexaphobia (fear of the number 666)

Hippopotomonstrosesquippedaliophobia (OMG! I REMEMBERED IT! )

pneumonia (or however you spell it...I think that's right though...not sure... )

psychic (I seem to always screw up the s and the p or add in an h somewhere...only when I'm typing though. My friend spells it physic )

plagiarism (I always put plajarism )