Best altitudes would be different for Naval and AF aircraft.
Most Naval engagements took place at lower altitudes, around the carrier and up to about 12 - 15,000 feet, at least in the PTO. I'd make best altitude for a Navy fighter around 18,000 feet with capability to get to 32,000 feet or so. They didn't have fuel to go for long escorts. They were for fleet attack and defense.
Most USAAF engagements took place higher, at least in the ETO. Somewhere around 20 - 35,000 feet. I'd make best altitude for a USAAF fighter around 23,000 feet with capability to get as high as possible.
Different missions. But, hey, that's just me and I wasn't in the decision making slot at the time.