MRI is still used in a lot of places, although YARV is rapidly becoming industry standard. YARV is the fastest overall. Although jruby has matured rapidly and is competitive. My preference is jruby atm. Rubinius is reasonably fast for what it does. Additionally, there's a lot of languages implemented in Rubinus as well. Twisol is implementing Lua in Rubinus, for example. You'd have to look at individual benchmarks to see what different implementations do faster or slower. They all have wheel houses. Currently, though, using yarv for rails 3 is the most common and "fastest" setup.