The team I used for both Diamond and Pearl (not going to use version exclusives):
Starter: Infernape, offensively, it's the best of the three starters, since it has good attack and special attack and is fast, early game is really easy, since it destroys the first and second gyms. Also, sweeps the Steel Gym in Canalave City, Ice gym in Snowpoint City and first Elite Four member Lucian's Bronzong, and Cynthia's Roserade.
Moveset: Flare Blitz, Close Combat, Grass Knot (really useful against the second Elite Four member and the Water/Ground type Pokemon), Stone Edge (for Flying types)
Flyer: Staraptor, probably the best flying sweeper you can ask for. You can get Starly really early game, destroys Fighting gym and Grass gym. When you finally get Staraptor, it deals massive damage because of Close Combat and Brave Bird. Helpful against the fire Elite Four member and Cynthia's Roserade.
Moveset: Brave Bird (got rid of Fly in Canalave right before Elite Four, then retaught it after beating Elite Four), Close Combat, Roost (for Brave Bird recoil), Return (since you get it early game, friendship level will be high by late game)
Raichu: for the Water gym, didn't need a Grass type because of Grass knot on Infernape, which wipes out Quagsire, so Electric would be best for Crasher Wake's Gyarados. You get Shinx early game, and its stats are good and all, better than Pikachu/Raichu, once it becomes a Luxray, but its movepool is trash, and Pikachu learns Thunderbolt at level 26, so, in my opinion, it was the best option. The Light Ball is really rare, so I just got a Thunderstone by Sunyshore. Also good against the bird trainers, swimmers, and Cynthia's Milotic.
Moveset: Thunderbolt (obviously), Brick Break, Iron Tail, Thunder Wave/Attract (your preference, either is good)
Steelix: For the last gym, and, with Earthquake, it can deal massive damage. The reason I used Steelix and not any other ground type(s) that are a little better is because I'm lazy. You can literally pick it up at Iron Island at a decent level, and you can train it with Riley to earn more exp., too. Also, it destroyed the Ice gym with Gyro Ball (from Veilstone Game Corner prize) or Iron Tail, but it's low accuracy. Also destroys the third Elite Four and, for Flint's Steelix and Loppuny, I used Infernape and Staraptor,
Moveset: Earthquake (obviously, you can get the TM after Canalave gym in Wayward Cave under Cycling Road), Double Edge (good with the ability Rock Head), Gyro Ball, Crunch
Weavile: Useful against the Elite Four, has great attack and speed, like the first two, destroys Lucian and Cynthia's Garchomp. You can pick up the Razor Claw in Victory Road to evolve it. Ice moves also good against the second Elite Four member.
Moveset: Night Slash (the move I used to demolish all of Lucian's Pokemon, except for Bronzong (used Infernape for Bronzong)), Ice Punch (from Shard Collector near Pastoria City) or Avalanche, which is also good, Aerial Ace (for Fighting types, you will most likely outspeed and, hopefully, ohko it before you get demolished), Dig (strong move for Fire, Rock, and Steel types)
Last slot: Your choice, but I only used five Pokemon because my last slot was my HM slave Bibarel. Not worth leveling up because of bad stats and not good in battle against gyms and the Elite Four. Used Bibarel as my surfer, too
Hippowdon may be a better substitute for Steelix, due to stats, but I find the Steel dual typing more useful in game. Also, I was too lazy to level up a Hippopotas at that point in the game. Same goes for Gible, it's obviously better, but training it to Garchomp is time consuming when Steelix can do the job just as good, without all that leveling up.