Land based fighters were extremely important.
The F6F was essentially reserved for fleet use, so their extensive use was not untill the carriers began sustained operations after the summer of 1943.
In the meantime, the Corsairs and P38's in the SW Pacific were in a daily battle with the IJA/IJN.
You can say that the Hellcat is what made the carriers invincable. But it was the Lightning and Corsair that destroyed the remaining "cream of the crop" Japanese pilots.
Not unimportant, but less important. The South and Southwest Pacific campaigns were a series of advances that led to the re-conquest of Philippines. Beyond that, landbased fighter supported landings couldn't get to Japan without seizing territory in mainland China (with US forces, a huge undertaking consciously avoided). Carrier superiority was necessary to finish things off, (eg. Mariana's B-29 bases, Okinawa, invasion of Japan itself: carriers), basic geo-strategic fact of the war.
Less capable landbased fighters could still have worn down the Japanese in the Solomons and NG campaigns just at higher cost, but excellent fighters were a more critical commodity for the limited deck space of carriers.
Also the scale of F4U combat was considerably smaller than F6F. F4U sdns saw little action between early 1944 (when the Rabaul campaign ended) and early 1945 (when assigned to carriers in part because underutilized), the critical phase of the war, a year of pretty constant large scale F6F ops downing very large numbers of carrier and landbased Japanese a/c. F6F's also did some fighting from land bases in the Solomons, and F4U's only entered service there in early '43 a few months before carrier operations started up again in earnest in summer, and flew alongside F4F's, P-40's (and P-38's) in Solomons in that period: credit for gradually attriting the Japanese air arms pre-1944 is shared among a number of types. F4U perhaps better plane than the F6F, but no way as important in the Pacific War, as it played out.
In scale of operation it's a little closer between P-38 and F6F but same issue, F6F campaigns were the ones which absolutely had to happen and where it was more important for the fighters to be superior plane for plane, whereas it can be debated whether the P-38's main campaign, leapfrogging up the New Guinea and eastern DEI to the Philippines, was even necessary to winning the war. However within that campaign the long range of the P-38 was a critical advantage: it set the distance by which each amphibious operation could advance from the previous under landbased air cover.
Looking at the big picture level of the war, I can't see a non-carrier fighter being named most important in the Pacific War. If another category was introduced for most important landbased fighter, then it would be P-38, not the F4U.
Joe