Well, after lurking around here for a long time I have to weigh in on this having turned some wrenches on round engines.

While the Merlin V-1650-3 is one of my favorite engines I would have to vote for the Wright Cyclone series.

Consider this:
The Merlin was rated at 1520 HP and developed 1400 HP at takeoff. Although it wasn't a serious issue the Merlin's weakness was that one bullet or piece of flak could put it out of action which is inherent in any liquid cooled engine.

The Wright R1820-65 developed 1200 HP, while the R3350-23 a double row 18 cylinder pumped out a staggering 2200 to 3700 HP depending on the model. The earlier model did have some dependability issues though.

Entering the scene a little too late for WW II was also the Pratt R2800 which was shoe horned into the F8F-2P Bearcat, pumping out 2100 HP and outperformed most of the early jet fighters.

All that said, I would have to go with the round engines first......and then the Merlin for inline.