Hmm... in game wise, I look at the Pokedex and pick 6 Pokémon I like and want to use. Then I use it. Like I'm restarting Pearl, which was my first ever completed game, to make it more competitive for myself. I'm now looking for Pokémon that I want to use. I won't use powerful Legendaries in my teams, although once in a while I'll break that rule (I'd be using Azelf in Platinum if I was going to finish it).
Competitive wise, I like offense more than the other playstyles. Hitting hard, hitting fast, weakening walls, tanking hits and healing up, and setting up late game... all fun for me. Which playstyle competitively do you prefer? Offense, Bulky Offense, Stall, Semi Stall, Weather Offense, Balance, Weather Stall, etc.