Which ocean is going to be swallowed by the Supercontinent Cycle, Atlantic or Pacific?
The pacific has already lost part its spreading center, north america has been pushed over it.. A few tiny non-contiguous plates like the  Juan de fuca plate is all that is left of the east pacific plate. in places like the san-andreas the north american plates has complete overridden the spreading center. 

In truth there are four possible predictions for the future: Novopangea, Pangea Ultima, Aurica and Amasia. two predict the atlantic disappearing one predicts the arctic and the final predicts the loss of the pacific. I think the latter two are more likely given what we see in the north america/pacific interaction but this is just opinion, the truth is we don't know a lot about how oceanic plates disappear or how spreading centers die because by their very nature they do not leave much evidence behind.