If I could be ....
A Pokemon - Togetic, I'd help spread happiness everywhere!
Gym Leader - I'd say Ghost probably, and I'd be one of the last gyms to face. The gym layout would be a large library maze where trainers appear from seemingly nowhere and there are booby traps all over the place. I'd battle with a team of Drifblim, Haunter, Dusclops and Banette. After the elite four was beaten, I could be battled again but this time I would have Drifblim, Rotom, Dusknoir, Banette, Spiritomb and Froslass. I'd give out the Spectre badge which would be either the seventh or eigth badge required.
Champion/Elite 4 Member - If I was an Elite Four member I would probably use the same team as my improved Gym team; Drifblim, Rotom, Dusknoir, Bannete, Spiritomb and Froslass.
If I was champion, I'd determine my pokemon with a roulette, that way facing me would be a different challenge everytime! The pokemon available for my use would be probably all i have mentioned before in the E4 section, Jynx, Luxray, Magnezone, Dragonite, Vileplume, Espeon, Togekiss, Abomasnow, Empoleon, Sableye, Lucario, Lickylicky, Drapion, Yanmega, Flygon, Salamence, Altaria, Ursaring, Ludicolo, Manectric, Arcanine, Steelix, Golem, Vespiquen, Exeggutor, Magmortar, Electivire, Tangrowth, Gallade, Octillery, Wailord, Blissey, etc, etc.... It'd be very interesting to say the least!
A Ranger - If it was one of the actual available partner pokemon from the game, it'd be Snover. Otherwise I'd probably use Smoochum, Oddish or Drifloon.
If your pet was a Pokemon - I'd have a Swablu and a Togepi. I'd probably get a Shroomish, Skitty and Skorupi as well. And maybe a Poochyena...oh and Bronzor...Kricketot...Smoochum, Oddish, Snover, Spoink, Natu, Drifloon, Duskull, Cacnea, Buneary, Combee, ledyba, Mareep, Finneon, Magby, Elekid, Cleffa, Igglybuff, Burmy, Snubull, Paras... heck I'd probably get all the basic pokemon!