The Raptor engine in development is also very efficient (full flow staged combustion of methalox). And while the Merlin series has been conservative in most as

The Raptor engine in development is also very efficient (full flow staged combustion of methalox). And while the Merlin series has been conservative in most aspects, Merlin 1D has a few unique characteristics, like the pintle injector (previously used in the Apollo LM), phased-shutoff (or whatever the spelling is, from Tom Mueller's interview from half a year ago), use of subchilled propellants and a very tightly manufacture process that makes it very cheap to produce. On top of that it's now the engine with highest thrust-to-weight ratio in history that has flown, despite being open cycle (gas generator). And not a single M1D failed so far.