German words and language patterns are always tricky and fun to learn, and Weltanschauung is one of my favorites. I also love the words trichotillomania, chthonic, Boeotia, eleemosynary, and cwm. Unfortunately, Visual Thesaurus does not have a few of my other favorite words: onychophagia, bewusstseinslage, and cywydd.
The name of a city in Poland, this is one of the trickiest words I've seen. On top of its weird spelling, this word has a weird pronunciation, too.
Other words with very difficult pronunciations and spellings are rachischisis, acciaccatura, eisteddfodd, groenendael, and taoiseach. VT does not have a few others like cynghanedd, brettanomyces, quaalude, and bowssen.