I didn't down vote it, but the difference between RP1 and Liquid Hydrogen engine performance is WELL known. Thus his assertion that SSME's are better, although true as comparison of rocket engines in general, is false when comparing LOX/RP1 engines specifically (since the SSME are NOT LOX/RP1), which the grandparent was clearly doing.

Similarly, citing the 850 ISP of the experimental NERVA engines is equally out of context.

I didn't see the LOX/RP1 qualifier when I replied (I was responding to the quoted sentence).

Let me qualify the SSME claim as "highest Isp/Thrust of any widely used engine". Prototypes or engines that haven't flown much IMHO aren't able to make this claim.

The NK-33 may beat a Merlin 1D, but what much do they cost to make and what is the reliability? If you make 100 NK-33s and 100 Merlin 1Ds, what's your total budget, and how many NK-33s will fail compared to the Merlin 1D?

One thing I get with SpaceX, is that their engines are not designed to push the performance envelope. They want them to be cheap and reliable.

Yes, the RS-25/SSMEs aren't "easily the most efficient rocket engines ever built". As noted below, there's NERVA.

And yesterday's launch illustrates why people are kind of nervous about using them as a way to take off from the planet's surface.