Piatti, Bernier and Drogba for me.
Di Vaio was good, but Drogba had a much bigger influence on the team. It's not in the same magnitude. Bernier went to a couple of all-star games IIRC. He was a staple on that team. He was a captain and a local star player. He's not a big international all-star, but he deserves to be in the top3. Too bad Klopas and Marsh (for most of his stint, although he started to grasp how good Bernier was later in the season) didn't see his value on the pitch.
You could make a case for Di Vaio and Ciman, both were very good. Ciman's first season was outstanding. Talent-wise, Dzemaili could be on the list too. He had a powerful strike. Too bad he left for family reasons.