All the data I've seen shows an RBC is the most efficient! They cost a little more to build but you get what you pay for.
What do you mean by "most efficient"? As far as foot print of the filter and bioload capacity, everything I have read points to a fluidized bed filter being the best. I have also read that a bio-reactor with kaldness media and other aerated moving bed biofilters are very good too.
I have not implemented either of these yet, so I am eager to hear about their pros/cons.
They cost a little more to build but you get what you pay for. 
