Well, if you're asking about the most-used by developers actively
hacking on and with Ruby, see below. If you're wondering about the
most widely-installed version, though, I would say that 1.6.8 is
almost certainly the winner, since that's what shipped with Mac OS X
10.2 and 10.3.

Most users of OS X may not know (or really care) that Ruby is
installed, but they almost certainly outnumber us hardy adventures
using it by a wide margin.

Lennon