Diego Costa
The combative Spaniard joined Chelsea in 2014, and banged in 20 goals as Jose Mourinho's side won the league. If he wanted to he would brush defenders aside and if he wanted to he would just annoy them, but it was an effective tactic and earned him a place in the PFA Team of the Year.
The next season, his struggles mirrored that of his team, as Chelsea slumped to 15th in the league by as late as December. He spent more time getting involved in skirmishes with opponents and less time scoring goals and would end up with only 12 for the defending champions, with more appearances than the year before.
With Jose out and Conte in, he rediscovered his form the next season, with another 20 goals as Chelsea romped to another title. Then came the infamous text from Conte saying he wasn't in his plans, and he was frozen out of the side before being sold back to Atletico Madrid.