Air - Raven, Great Horned Owl, or white/grey Gyrfalcon, depending entirely on the location and times of day we'd be flying. Raven would definitely be the most practical of the lot: big and clawsome enough that nothing short of larger falcons want anything to do with them in the air, ordinary-looking enough to be inconspicuous at most temperate latitudes, at most times of day. Weather hardy, frequently seen both solitary and in groups, so not a giveaway if we fly solo. Soaring and woodland gliding capabilities, both, and just migratory enough to handle distances well.
If we're dealing entirely in night flight, the owl. If we're in cold latitudes with enough snow cover to keep us from being conspicuous, falcon.
Land - Least weasel (Mustela nivalis). Tiny, great for hiding, seasonal camouflage, and ten types of vicious when harassed. If by 'land' we mean 'combat', then Polar Bear.
Marine - Leopard Seal (if cold water). Otherwise, Shortfin Mako Shark. Fast, migratory, able to leap incredibly high from the water... and the whole 'has gills' thing is useful, too.
