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I start my team with pokemon I really like, and then fill out the rest slots with pokemon that help it out. More often than not I use mono type team or old favourites to round out the team.
Of course, since I like couple powerful pokemon (Scizor, Heracross and Volcarona from my Bug team are OU), it doesn't mean that my teams aren't inherently terrible just because I use favourites. My Poison team is similar in that way (Crobat, Gengar, Scolipede, etc).