I guess it's more a topic of ''of all 18 Types, which one is your favorite''. Whatever! I'd like to know what type you'd be using, if you were a Gym Leader, or forced to have just 1 type! You can give reasoning if you wish to.
I go with Water!
I just like it! Of the 4 classical elements (Fire/Water/Earth/Air), it's already my favorite. Water pokemons might not be the strongest, but they certainly are a match of their own! Plus, most can learn Ice moves as well, to deal with Flying, Grass and even Dragon types! Finally, I can name quite a few who have a side-type as well (Empoleon's Steel, Starmie's Psychic, Gyrados's Flying and Lanturn's Electric comes to mind), so you can still come up with different moves types for different situations. Heck, I just looked up and Marill might be Fairy secondary, too! :p
So, yeah, that's my reasoning for choosing Water, were I Gym Leader! What about you guys?