In order of difficult for me:   Malenia blade of Miquella,  Twin Gargoyles in Siofra Aquaduct,  Malaketh the Black Blade,  Radahn and Elden Beast maybe tie for fourth I'm not sure.

Ridiculously easy:  Ser Gideon Ofnir the All-Knowing,  Renalla Queen of the Full Moon.  Both killed first time without even knowing how they fight.