In an odd Platinum run-through, for some reason I nicknamed my Pokemon after Batman characters. Batman the Gliscor, Penguin the Empoleon, Joker the Magmortar, can't remember the rest. I think I had a Gallade named Riddler. But in normal playthroughs of a Pokemon game, my team includes:
Fire - If my starter is Fire, then I use my starter. Usually I use a fire type I can get earlier in the game. Because you can rarely get a fire type early on, I usually just pick up whatever I find first.
Water - Like Fire, this could be my starter. But I don't generally use water types that you can only get late in the game. With the exception of Chinchou/Lanturn in RSE. If I don't start with Mudkip, I often use one of these.
Grass - Like Water and Fire, this is my starter at times. I prefer double-type grass Pokemon, and I don't always have a grass type. I find it useful sometimes, though.
Electric - This is a team necessity for me. I never use a Pikachu though. In Kanto games, I just use a Magneton. In Hoenn, it was Manectric. Or Lanturn, like in Johto. I didn't raise one in Diamond, but Platinum (Not the aforementioned Batman playthrough) brought me Electivire, which I used. My first 2 Unova playthroughs, I used Zebstrika and Galvantula, and now, in my 3rd playthrough, I am going to use an Eelektross.
Ground/Rock/Steel - A defender is necessary for me, and I rarely have trouble finding one of these. Golem appears often in this role, he is very underrated and his defense is great.
Whatever - Whatever I feel like raising goes here. Sometimes it is a flying type, sometimes something different like Dark, Ghost, or Psychic.