No question about the ranking of importance *to the tactical system*: Messi, Suarez, Neymar.
It's easy to sum up Neymar's most important contribution: good in-behind runs and clinical finishes in some games where those finishes were important. Second, some will say that his presence on the left touch occupies defenders fearful of his attack. I personally am not convinced, as I see him as usually ineffective except when around the 18 yard box.
On the other hand, Suarez center/ Messi right *is* the creative engine of this attacking system. With the false 9 rarely effective except in certain situations, having a dangerous creative #9 has been key to this new Barça's success, whether or not Suarez gets on the scoreboard. He is important in every game he plays. He occupies the centerbacks, giving Messi room to work his magic (including countless crosses to Neymar at the back post). Inside the box, Suarez is second only to Messi in creating a goal out of absolutely nothing.
All have executed their roles well, but it's Suarez, hands down, for me. If I had to give up one of the three players, I'd be keeping Messi and Suarez.