I'm going to have to agree with the guy who mentioned Zerbinetta's aria. It seems to be the hardest.
Here are some others that are VERY difficult:
Glitter and Be Gay - Candide
Martern Aller Arten - The Abduction from the Seraglio
Big Brother - 1984 (by Lorin Maazel. There used to be a video of Diana Damrau singing it on Youtube, and it really sounds like a vocal cheese grater)
Caro Nome - Rigoletto (difficult due to long lines and a varied tessitura - both higher and lower)
The Bell Song - Lakme
The Doll Song - Les Contes D'Hoffman
Da Tempeste - Giulio Cesare
O Luce Di Quest Anima - Linda di Chamounix
The Mad Scene from Lucia di Lammermoor
Sempre Libera - La Traviata
Lo Hear the Gentle Lark - Art song by Bishop (unBELIEVABLY difficult)
Poor Wandering One - Pirates of Penzance (can be difficult with the right ornamentation)
A Vos Jeux/Pale et Blonde - Hamlet
Ah Non Giunge - La Sonnambula
Bel Raggio Lusinghier - Semiramide
Sovra Il Sen - La Sonnambula
The Russian Nightengale/Die Nachtigall/Le Rossignol by Alabiev
Una Voce Poco Fa - Il Barbiere di Siviglia
Regnava Nel Silenzio/Quando Rapito in Estasi - Lucia di Lammermoor
Those are just a few. There are loads of difficult coloratura pieces out there.
Hope this helped!