When I think of the "ultimate" running shoe I think of the Roclite 315. There are few things that are absolute essentials when I am running and racing, but I can attest that I have found running nirvana with the 315's over the last year.
Like all Inov-8 models, the Roclite series has the light and comfy feel of cushion for those long hauls, yet allows the feet to perform naturally. The 315's have great traction for a variety of different terrain, whether it be mucky muck, gravel roads, or rocks and ruts. Another great thing about the 315's are their versatility. Last year I wore these shoes in distances from 5km, 1/2 marathon, 50km, 50mile and 100mile distances. The Roclite 315 is a true iron horse that can go the distance and keep the feet happy (: