I can't really decide on which type I like the most, though I always end up using at least one Water type in my team. If I were a gym leader, I would be Water type, using the following: Kingdra, Gyarados, Sharpedo, Tentacruel, and Jellicent. I chose the following because they have a nice theme to them (sea monsters). So, following this theme of sea monsters, I would have my gym underwater, requiring like Dive or something to get to it. The gym would be inside a sunken ship, and prior to entering the gym leader's arena, the trainer will be given an oxygen tank. The gym would be set up in 3 layers. the first layer will be sandy and plain, representing the most shallow part of the ocean, and will have 1 trainer blocking your way. Upon defeating that trainer and entering the second layer, the layout will morph into a coral garden, and will have 2 trainers blocking your way. The final layout will be a deep sea theme, with underwater volcanoes and faint dim lights to guide your way into the pitch black ocean floor. There will be 3 trainers to face there. And finally, the trainer will reach a slope going even deeper, until the whole screen is pitch black. As you go deeper and deeper, you will see a faint light in the corner, and I'll be there.
My losing message would be "Just when I've reached the deepest area, I must rise to the surface and retrieve a badge..." Upon exiting the battle, both the trainer and I will float upward gradually until we reach the surface, until both our heads exit the water. I will then give the Abyss Badge to the victor.
Sorry for wall of text, the topic interested me ^.^"