For fighter to fighter combat on a real WW2 airplane, nothing beats the combination of 4 20mm MG 151s in the wings and 2 13.2 mm MG 131 in the cowling of a Fw190A-6 or later.
Close runner up would be the Tempest with the Mk V Hispanos with increased rate of fire and 200 rpg.
The Beaufighter and Mosquito also deserve special mention. The fighter bomber versions of the Mossie mounted 4 Hispanos with 300 rpg and 4 .303s with 3000 rpg! The Australian versions of the Beaufighter had 4 M2 Brownings with 500 rpg and 4 Hispanos with 250 rpg. British versions had 4 Hispanos and 6 Browning .303s!
Technically speaking, I prefer the British/Soviet approach of high velocity, flat trajectory cannons with a high AP and moderate HE value. Easier to shoot at small targets than with lighter shells, much better terminal and AP effects than a HMG and similar MV and ROF to a HMG.
So, my ultimate single seater fighter armament would probably be a FW-190A equipped with 4 Hispano Mk V in the wings and a Soviet UBS Heavy Machine gun in the cowling.
For bomber interception nothing can really better the He-219 'Uhu'. The A-R/7-1 had 2 Mk 108s in the wing roots, plus two more and 2 Mg 151/20s in the gun tray. It also carried 2 more MK 108s in a Schrage Musik installation. Total of 6 Mk 108s and two Mg 151/20s!
Fighter armament
1st Hispano Mk V 20mm
2nd Berezin B-20 20mm
3rd UBS 12.7mm
Bomber armament
1st Mk 108 20mm
2nd MG 151/20 (especially with MG shells)
3rd Hispano Mk V 20mm