Cook is good, but honestly, nowhere near the skillset of Kittle. Kittle is almost impossible to cover with a LB, heck - even corners and safeties struggle with his size and route running.
More accurately, we have someone like Kittle in our offense - Michael Thomas. Basically a guy who can't be covered in the intermediate routes and can make people miss once the ball has been caught.
But Kittle invites the contact and like a prime Gronk, can fight off tacklers.
Again, Cook is good but he can't get open like Kittle and he can't shake off tackler like Kittle.
Kittle is the best, Kelce is a tier below and everyone else is in a battle for 3rd.