Time to show off my current Pokémon Diamond party. Beware, that this is definitely not my main party, or my final one.
Staraptor Lvl. 61 Brave Bird, Close Combat, Steel Wing, Fly
Floatzel Lvl. 59 Crunch, Razor Wind, Waterfall, Surf
Dialga Lvl. 60 Earth Power, AncientPower, Dragon Claw, Roar of Time
Bibarel Lvl. 30 Strength, Rock Smash, Rock Climb, Cut (My HMs-only Pokémon)
Luxray Lvl. 59 Spark, Discharge, Thunder Fang, Crunch
Pupitar Lvl. 44 Dark Pulse, Rock Slide, Crunch, Thrash
My Torterra (Lvl. 62 with Crunch, Earthquake, Frenzy Plant, and Leaf Storm) is in the PC to make room for Pupitar, who will be in my final party as a Tyranitar I think (I have one in Golf that I love so much, I have to have another). The above Pokémon (minus two), as you'll see below, will not make it to my final party, but I've 1.) Been with them for a long time and have attached to them, and 2) They make a good secondary party. The following Pokémon, I believe, will be in my final party:
90% GUARANTEED
Torterra
Dialga
Heatran
Giratina
POSSIBLE
Machamp
Darkrai
Spiritomb
Tyranitar
The guaranteed ones are all very strong and can be very helpful to my party; Torterra with it's Defense and overall effectiveness, Dialga with its high and well-rounded stats (Can't go wrong with Dragon or Steel, but together = WIN), Giratina and it's Shadow Force (Also, Dragon and Ghost both great, but together = WIN), and Heatran with it's Fire/Steel type, providing both good Attack and Defense.
The possible ones are also good considerations. Machamp has a very fitting Pokédex description: the Superpower Pokémon, as its Attack goes off charts (300s+). Darkrai has its very useful signature attack, Dark Void, which puts its opponents to sleep and sucks away their health. Spiritomb has no weakness and has somewhat high and well-rounded stats. Lastly, I just love Tyranitar. It looks freaking awesome, has good Attack and Defense, and can be a very strong Pokémon to have to go up against.
Hopefully, in a few months, I will have my final party all figured and and leveled/gathered up. Wish me luck, Pokétrainers.