Well, a bent-wing guy gotta step in here (all comments apply to 1/48 scale):
F4U-1, -1a, -1d and F4U-2 - The best on the market is the Tamiya kit in any of its incarnations. Hobbycraft (and Academy clones) are ok, fairly good in outline but lacking detail. Arii/Otaki Corsair kits are good in outline, but with weird details.
F4U-4 - For the best representation, use an Tamiya -1a or -1d with a CMK conversion. Hasegawa's kit is pretty good for having raised panel lines, but the wheel wells are shallow, and the windscreen frames are too far apart at the top. Academy's has recessed panel lines, but the fuselage is distorted around the cockpit, resulting in a mis-shapen canopy.
F4U-4b - Hasegawa just released their -4 with sprues from the -5 for the 20mm cannons and HVARs. Gun covers and ejection ports are still for the 6 x .50cal version, however. Haven't built Academy's offering, so I can't comment.
F4U-5, -5N, -5NL - Hasegawa has the best offering. Its not perfect, but finishes up nicely. Minicraft's offering is a joke with no redeming features.
F4U-6 (AU-1) - Again, Hasegawa with modified release of their -5 family. Both USMC and French Nave boxings are offered, and, Hase really nailed the shape. Heller's offering missed the shape of the fuselage, although the decals are really nice.
F4U-7 - Hasegawa, once again. The Heller kit just doesn't have anything to offer other than good decals.
Just to clear up some things: The F4U-5 was essentially an all new airframe, which was about a foot longer forward of the wing to accomodate the twin-scroll superchargers. The lengthened forward portion was distinctly "pear-shaped". The AU-1 used the basic -5 airframe, but with enhanced armor and without the high altitude engine. The French liked the AU-1 so much, they ordered their own version (F4U-7) which reverted to a high altitude engine (in this case, the same engine as the -4) and retained the longer, pear-shaped -5 fuselage.
Bent Wings and Round Engines forever!
Phil