The whole thing
But if you want me to narrow it down
-Channel Front 1943-44 with the Spit XIIs of 41 and 91 Squadrons
-Southwest Pacific with the 38 drivers of the 80th, 39th, 475th, etc along with the 5th AF medium bomber ops in mid to late 1943
-The 1942 airwar of the 54th FS and their P38Es in the Aleutians
-The MTO airwar of the 1st, 14th and 82nd FG P38s
-The 1944-45 tac air war of the 9th AF P38s in the 474th FG.
Oh and I've always been a big fan of the 4th FG and their P51B/C Mustangs in the spring of 44 when the airwar was really hot over Germany.
Argh! Also the 454th BG B24s of the 15th AF and the B17s of the 447th BG 8th AF having researched two crews, one from each for the families of their lost family members.