In the ETO
P-38 = 130,000 sorties
P-51 = 214,000
P-47 = 404,000?
I'll try to confirm those if I can remember where I picked them up at.
The PTO is harder because records destruction was not uncommon.
If I were to guess though it would be the bf-109 with 36,000 +/- built and multiple sorties/day for most of the war. part of this is determined by planes produced/available ie.
P-38s 10,000+/- over the duration of the war used simultaniously in all theaters (except Russian),
P-51 15,000+ built from 43-45 Primarily used in the ETO. Started spreading out in late 44
P-47 16,000+ built 42-45 primarily used in the ETO but spread out earlier than the P-51.
bf-109 36,000+ from 37 to 45 primarily ETO but also MTO.
Spitfire 22,000+ from 39 to 47? (I saw these numbers in an Airforce Identification book I had in the 70's. I'd die for it now 3D pics. and up to date official specs. Priceless) PTO, ETO, MTO.
I think only 4 fighters were built the entire war P-38, bf-109, Spitfire and Zero.