Cheese on Bendtner, ffs.
There are a couple of different "right" answers, depending on how you look at the question.
In business terms you are considering which player will give you the biggest return over time, so you're thinking about transfer value increasing as well as productivity on the pitch: only Coutinho, Kane and Lukaku are young enough to fit that description.
In terms of pure "on the pitch" contribution... Zlatan. He has an aura about him. He's a menace, and a leader. But given his age, and the ACL, I wouldn't pay a transfer fee for him. He's a "missing piece of the puzzle" transfer for a team looking to win the Prem, or the CL.
Costa is a weapons-grade bell. The type of signing that would have me question my future support of the club. Only slightly less unacceptable than Suarez.
Sanchez has a worse record against the top 6 than Lukaku and Kane. Coutinho also doesn't measure up in that regard. Both are still great players though, I like them both. I also like Aguero a lot, but he seems to pick up injuries more often than his contemporaries.
Voted Lukaku. More assists than Kane, all his goals are from open play (unlike Kane) and he's done it with lower-quality support. I can see why people rate Kane though.