I'm assuming there isn't a word that contains every letter in the alphabet, so which word contains the most?
antidisestablishmentarianism - 12
psychotherapy - 12
handcraftsmanship - 13
Among words in a certain word list I found:
It has ten words with 16, including:
(16, 'superacknowledgement') (16, 'pseudolamellibranchiate') (16, 'pneumoventriculography') (16, 'phenylthiocarbamides') (16, 'hippopotomonstrosesquipedaliophobia') (16, 'formaldehydesulphoxylic') (16, 'blepharoconjunctivitis')
There are several dozen with 15, including:
(15, 'undiscoverability') (15, 'uncopyrightable') (15, 'supermagnificently') (15, 'supercalifragilisticexpialidocious') (15, 'submetaphorically') (15, 'spondylotherapeutics') (15, 'semiautobiographical') (15, 'pterygomandibular') (15, 'pseudoromantically') (15, 'overdiscriminatingly') (15, 'neuropharmacologists') (15, 'hyperconstitutionalism') (15, 'honorificabilitudinities') (15, 'gumpoldskirchner') (15, 'formaldehydesulphoxylate') (15, 'encephalomyocarditis') (15, 'dermatoglyphics')
and several hundred with 14 (including a few 14-letter words like 'pseudomythical', 'hydropneumatic' and 'ambidextrously').
In case anyone cares, this was done with Python, something like:
F = open('words.list').readlines() for i in range(len(F)): w = F[i].strip().lower() F[i] = (len(set(w)), w) F = sorted(F, reverse=True) for i in range(len(F)): print F[i]
A better word list (one that includes subdermatoglyphic, for instance!) may give longer words. And you didn't ask about two words, but just for reference: this says "Ralph Beaman proposes formaldehydesulphoxylic ventriloquizing as a two-word combination having 22 different letters which should be difficult to beat"; I haven't checked this.
Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch clocks in at 18 distinct characters, but arguably a near-infinite number of 25-character answers exist, since organic chemistry nomenclature allows one to build valid hypothetical compound names of essentially arbitrary length, and the full alphabet less only J appears on the periodic table (though this does require one to refer to rutherfordium by its old placeholder name unnilquadium or arrogate a placeholder name for a transuranic element with an atomic number ending in 4 that hasn't been named yet, so it may only get us to 24 characters).