Is it possible to answer this question?? Not for me. I 愛 every one of her songs. They are all too catchy, and addicting! :) If I had to say one, I would say... Umm.... あなた and I is one of my favourites.
Judas, Summerboy, I Like IT Rough, Telephone, Monster, Bad Romance, Just Dance, 愛 Game, Poker Face, Electric Kiss, ......... That's all I can think of right now..:P