Ok, I beat the game, and I'm making a fancy new team because the one I used in game kinda sucked....
I came up with 5 pokemon I wanted on my team... I need a sixth pokemon. I won't post movesets, since I haven't thought of them yet, but you can expect the pokemon to have STAB moves.
So far the team is...
Gengar
Machamp
Togekiss
Garchomp
Metagross
I was sorta leaning towards using my Kingdra, or a Magmortar, but I dunno....
Any pokemon in SoulSilver or Platinum is good. But not from the Safari Zone in Silver. I hate that place.
Oh, and should Garchomp know Dragon Claw or Dragon Rush?

garchomp should have outrage it is far better

I pick pokemon by their types and what types I've got already.
Kingdra is a water/dragon and only weak against dragon which makes it perfect against grass and ground enemies.
However since you have Garchomp you already have an advantage against pokemon weak to dragons.
Magmortar is a single fire which gives it multiple weaknesses. ground type attacks are not used often unless it's Earthquake and water pokemon are very common in battles so you might need a pokemon that can handle water types.

Gengar isnt that good, im my opinion,

Machamp, make sure it has no guard and an ability and stone edge
Togekiss, Classic flinch set works for me mabe you too (serene grace with t wave, airslash double team aurora sphere)
Garchomp, any psycial sweeper set
Metagross, good pysical tank

Based on your team id sugest Milotic, bulky wter and a trype your missing, surf ice beem combo aswell as aqua ring and mirror coat is always good
Im my opinion id replace gengar with charizard, Sunny beam combo + flame thrower, aslong as garcomp has crunch and machamp has thunder punch (to replace gengars presumed engery ball water counter) it should work fine

Defintiely not the Kingdra, double typing is pretty much a big no-no.

I would go with either Electrivire or Magnezone