F4F-4 saw action in June 1942 aboard USN CVs (Midway). The F4F-3 (4 gun version) was in service on CVs December 1941, and saw action at Coral Sea before Midway.
FM-2, as stated, deployed to the Pacific in large numbers in the Spring of 1944 on Escort Carriers. First kill March 1944. Was used by the USMC, the USN, and the Fleet Air Arm (In the Atlantic).
F4U-1 was deployed with the USMC to the Pacific in February of 1943, it scored its first kill with a Guadalcanal based VMF the same month.
F6F-3 Hellcat was deployed operationally to the Pacific aboard USN CVs in August 1943 (CV 10, USS Yorktown). The F6F-5 was an updated version, but not in a big way, I beleive it had some extra armor and wider variety of A/G ordnance. Air to air, they were very close, with almost identical performance.
The biggest mistake most folks make is thinking that the F6F was in action at the same time as the F4U. The F4U was in combat 6 months earlier at least. On the other hand, it wasnt untill late 1944 that the USN started operating the F4U from its Carriers, prior to that they were land based only, most with the USMC, and some with the USN (most notably VF-17).
Hope that helps a bit.