It's probably bad to evaluate Novak on grass based on his performance on a bum knee, but it does seem like RG is the least competitive.
Wimbledon has Raz and potentially Sinner when fit
USO is out of the question unless he gets a draw like 2023 where all threats are in the same half
AO is his best Slam, but all the other 4, Raz, Med, Sinner and Zed can be very tough there
RG only has Sinner, Zed, which Novak can outplay and Raz, which he might beat on a day similar to today.
But at any rate, his best chance will be when he is fully fit, prepared and has an easy draw, not at a respective Slam.