No definitive answer perhaps, but what is your personal guess?
Which Vala do you think caused Gollum to teeter on the edge of the Sammath Naur?
Eru himself would have commanded it, perhaps, but to whom might he have delegated the task? And how many of the Valar were watching, and how many of them exerted their powers in those final minutes? Who put it into Frodo and Sam's minds, as they rested on the slopes of Orodruin, that they must get up and push on to the Cracks of Doom, before it was too late.
My guess would be Manwe, perhaps (regarding Gollum's sudden loss of balance). Ulmo seems to intervene more than most in the fates of the peoples of Middle-earth, but I'd find it difficult to imagine him keeping watch around a parched, waterless land. Unless he had taken it upon himself to watch from the Sea of Nurnen.
Et vous?