The effective fly time between two complete revisions of the engine can be important. I read that's of 48 hours for R-2800 and 50 hours for DB-605. Someone know what's for Merlin (first versions of which are not so famous for reliability however)?
I think even soviets M-82 / ASh-82F (radial) and the Mikulin AM family, from 35 to 42 (inline), could be considered.
I would wonder about the R3350 though. It had reliability issues as compared against the other engines through most of its useage in WW2. It really didnt have the bugs worked out of it untill after the war ended.
Don't forget the 1800 and the 2600 which flew thousands of missions during the war. Not as powerful as the 2800 but just as reliable.
And for liquid cooled engines the Allison was as reliable as any other including the Merlin. The problems with the early P-38 in the ETO (only)have not only been paraded in the press but are totaly blown out of proportion.
The V-1710 offered a high level of reliability in the later versions of the P-40. The RAF found out that an Allison could eat quite a bit more sand than a Merlin, an important feature in North Africa. In the Lightning, the V-1710 required more maintenance, but that was due to turbocharging. I would still consider even the P-38's Allison's reliable, and wouldn't say the turbocharged V-1710's were as troublesome as the R-3350's were during the war. The Junkers Jumo was also noted for it's reliability.
Also if you read my post, you will see that I said nothing about it being "saw widespread us" at all. I said that this was my List of the top engines of WW2 not the Top Engines of WW2 to See Wide Spread Use list.