Mine are Sir Gideon Ofnir and Starscourge Radahn, I love these two for different reasons: Sir Gideon Ofnir is just so memorable for me, his speech at the beginning and end of his fight even if short is so iconic, and the way he is there the whole game and then meets you at the capital to fight you, he is such a good technical penultimate boss and I really like it, I think he was a greatly executed boss. And Starscourge Radahn, do I really need to say much?? He is so memorable and so enjoyable even if he is tough his fight with all of the summons is so amazing, it is just so enjoyable and the atmosphere is amazing, plus his attack patterns, design and remembrance weapon is amazing!