If I could be...
a Pokemon: Golurk. I'd get to keep my opposable thumbs while being capable of high-speed flight and virtually indestructible, and I'd get to be my second favorite Pokemon, after Jirachi
a Gym Leader: Ground. I'd be an oldish, semi-retired, Indiana-Jones-style archaeologist who runs one of the region's largest, most challenging gyms in the region inside the mostly-intact, semi-renovated ruins of an ancient fortress on top of a mountain. I would have a team for each of the eight badge challenges, with at least half of the Pokemon on each team being evolutionary relatives of something on my ninth, final, strongest, "rematch" team. I would specialize in the Trapinch and Golett lines, having at least one member of one of those lines appearing on every team, and the ruins would be filled with friendly reactivated Golett and a pack of wild Trapinch, Vibrava, and Flygon that took up residence there in the period between their fall and their rediscovery. I would give out a TM for Earth Power, which all of my Pokemon would know.
The teams:
1st: Trapinch, Golett
2nd: Gligar, Trapinch
3rd: Marshtomp, Golett, Sandshrew
4th: Nidoking, Sandile, Golett
5th: Torterra, Gastrodon, Vibrava, Larvitar
6th: Golurk, Excadrill, Gabite, Gligar
7th: Flygon, Gliscor, Claydol, Hippowdon, Krokorok
8th: Flygon, Golurk, Steelix, Camerupt, Sandslash
Final: Flygon, Golurk, Gliscor, Torterra, Sandslash, Nidoqueen
Badge: the Ruin Badge
Ranger partner: Reuniclus. It can levitate to hard-to-reach places, heal injured Pokemon with a Pain Split coming off of base 110 HP at at least level 41 (if you want to go by game mechanics for this sort of thing, anyhow) and then heal back its own health with Recover, crush boulders with their psychically-controlled arms, and possibly able to communicate with humans, seeing as how it is described as "remarkably intelligent," and said to be able to link its mind with other Reuniclus, which means it might be able to be trained to do the same with humans and injured Pokemon. It's also one of my favorites.