僕は 彼女*が*大好きです is perfectly correct, but not because there's an object there. It is because" watashi wa NOUN ga daisuki desu" is a grammatical construction, used very often and very common. For example, watashi wa kuruma ga daisuki desu, or, watashi wa kuruma ga kirai desu, or, watashi wa kuruma ga hoshii desu. But it still would be kuruma o uten suru not kuruma ga uten suru, it's just that these three verbs (kirai, daisuki, and hoshii) are used with particle ga, not with particle o.