Merry or Pippin

[The Might]

Merry definitely did more to influence the War of the Ring...
In the movie you might be decieved since you see Pippin light the beacons, but in the books it's clearly Merry that does more...
Pippin seems to see it like that as well because as he is sitting in front of the Black Gate with the Host of the West he says to himself:

'If only I could smite that foul Messenger with it, then almost I should draw level with old Merry. Well, I'll smite some of this beastly brood before the end. I wish I could see cool sunlight and green grass again!'

Pippin did indeed play an important part in saving Faramir, but this was not so important for the outcome of the War of the Ring, since Faramir would no longer play an active part in the events. It was indeed his foolishness that brought a great deal of luck to the Fellowship since he could see a part of Sauron's plan in the palantir.

Still, Merry was clearly more important as he was the one who made it possible for Eowyn to kill the WK. Had this not happened, the events that took place afterwards might have been very different.