cat druids will top them all, but enh dps will generally be a pretty close second (closely linked with boomkin and elemental).
the core idea behind this being druids in cat form cant heal. elemental shaman and enh shaman can. thus, cat druids are the more "pure" class, and have the stronger base ability to DPS.
its really a wash if you think about it tho; any top-notch melee dps group has an enh shammy and a feral druid in it, as well as one warrior and either a rogue or a hunter. the synergy of the buffs in that group far outshines any personal dps contribution. in terms of non-raid settings, balance, elemental, feral, and enhancement all kill with pretty equal speed.