I always go for a balanced team that can take on any combination of enemy types. There have been times I was somewhat unbalanced for the sake of using Pokemon I desperately wanted to, and that hasn't always worked out very well.
Playing) Fire Emblem: Awakening, Guild Wars 2 Waiting) Pokemon XY
I guess it depends on what you consider a "traditional team" set-up. I always use my starter Pokemon (always either Fire or Water, never Grass), an Electric Pokemon if possible, a Pokemon who can learn Fly, a Pokemon who can learn Surf, and a Dragon of some kind. The last Pokemon depends on which Pokemon are available to me in-game; it just has to be not completely useless and have some type-coverage. Here is an example of the Pokemon I used in my play-through of the story-mode of Pokemon Black 2:
All of those Pokemon were available to me throughout the game without having to trade and were able to get me through the Elite 4 and Champion. I used a similar set of Pokemon in my last Platinum run as well:
The only exception was gen I when there was no dark. And the setup pretty much stays into my competative team except Espeon may be getting replaced by Sylveon if it has good enough stats and can learn psychic via tm.
Stop teaching kids if you're happy and you know it! ...It hurts timmy the T-rex's feelings when he cant clap like the other kids...