Legendaries.This is my opinion but i would chose Darkrai,Arceus,Regigigas,Mewthree(yes its a real obtainable pokemon),Lugia,and Ho-oH
BYe bye bye
The best Pokemon to have in your party is DARKRAI. If it is LV.100 you can't beat him. The best moves for DARKRAI to have are Dark Void, Psychic, Focus Blast, and Dream Eater.
There is no specific "best party" but to get a great party you just need to have a party that consists of the main components. You need to have a variety of types, an attack Pokemon with a remarkable attack stat, a "wall" Pokemon which should be able to take as many hits as needed (can either have high DEF, SP.DEF, or HP) and a secial attack Pokemon.
Fire and water are the best combination to beat the elite 4.
for your party, you'll need Pokemon that know most of the HMs as they are used often
In pokemon platinum, the best party ever is Infernape, Staraptor, Luxray[third evolution of Shinx], Floatzel, Bibarel [make it learn all its moves as HMs] and Giratina.
torchic - absol - spheal the regis - allways leave a empty space in your party.
Your Pokemon Party are the six Pokemon which you take with you
In my opinion, the best party with move sets and EVs in Pokemon X including the legendaries is the trade and using of cheat codes such as AR.
What do you mean 'how do you party pokemon'
there isn't a "best" Pokemon team but mine is: Blaziken lv76, charizard lv72, alakazam lv70, electivire lv73, sceptile lv72 and my last Pokemon is a swampert lv74 =[)
arceus,ho-oh,lugia,groudon,dialga and palkia