What teamshould i aim for

[DrLems]

So ive played all the pokemon games from gameboy colour > old ds, but never played x/y cause i had no 3ds but im finally getting one on the 28th with omega ruby and ive been reading around alot on here about teams for championships etc but just curious, what would be a good team to aim for, as my starter team im planning on going into online battles alot because im quite competitive but just not sure where i should start xD all i know is id love to have gengar in my team! Thanks <3