I'm really torn between what type of Pokemon I'd want to use. I love Poison-types and always will, but I've absolutely fallen in love with Fairy-types too. I'll do potential lists for both types then.
But first, on average a gym leader will have three or four Pokemon in their team, and many of them are repeats or the unevolved versions of their final Pokemon. They also usually have a "signature Pokemon" on their team too. I'll write those in bold;
Poison - Koffing, Koffing, Weezing, Drapion
Fairy - Marill, Floette, Floette, Azumarill
Of course those would be my teams if I were a genuine Gym Leader in the games. I'd certainy get more crazy for more competitive battles, and I do when I build mono-type teams for fun. I had a mono-type Poison team in Gen IV that was Weezing, Venomoth, Swalot, Croagunk, Qwilfish and Drapion and when I was into Ice-types my team was Castform (knew Hail and some Ice moves as well as Weather Ball), Dewgong, Shellder, Jynx, Glalie and Weavile.
My Elite Four teams would be similar to what I've put for my mono-type teams kinda but of course Elite Four members only have five Pokemon on their teams and my Poison team would be updated to include Pokemon from all six generations. Again my signature Pokemon are in bold;
Poison - Weezing, Amoongus, Qwilfish, Dragalge, Drapion
Fairy - Flogres, Sylveon, Klefki, Togekiss, Azumarill
Just for fun I'll do my Champion team. If I was a Pokemon Champion I'd definitely have a mixed team consisting of six Pokemon;
Pinsir, Druddigon, Gourgeist , Drapion, Azumarill, Aggron (Mega Aggron)
Probably not the strongest team in the world and there are some definite flaws, but I love those Pokemon, so yah. If I was given a little more time to think my team through I would definitely pick some different ones, but that's how it would stand as of now.