The R-2800 'A' gave some 1200 HP at 2600 rpm at 20000 ft, military power - some 100 HP more than BMW 801C on 2550 rpm (3 min rating). Both were heavy engines, the BMW was less bulky, with better exhaust stacks layout. R-2800 was more reliable and gave much more power under 15000 ft.
The Merlin XX is top contender, with favorable power at all altitudes, top marks for reliability, far less bulk and weight than radials, favorable consumption.
DB 601N is not as powerful as the Merlin XX, it's 1 or 3 minute max rating is lower than 5 min rating for Merlin XX. DB 601E is in race, but it was not allowed for Notleistung until Dec 1941? 601E can have gun through the prop, and can use low oct fuel, the consumption is a bit better than for the Merlin. I'd still rate the XX above the 601E, in this year.
How about Hercules? The Mk.IX does 1500 hp at 12000 ft (~1050 at 20000 ft), and 1575 at 500 ft. Not as good at altitude as Merlin or R-2800, but it is good engine under 15000 ft, and not that heavy and bulky as R-2800.