Counterpick would really depend on the opponents.
Low mobility (IE Snake, DK, Bowser): Rainbow Cruise
Poor recovery but nimble (IE Olimar, PT, Marth, CFalc): JJapes. The water will drag them to death whereas GW and ROB both deal with water well.
Agile (MK, Diddy, Ect): Frigate [Note: Need to know how to fight in close quarters with your team mate without hitting him too much]
If there's a combination of two of those types, it's really a coin toss for me.
As for neutrals, I'm not sure where GW is good on the neutrals so I can't really help there. I'd avoid FD and Smashville though because I -think- GW is bad there.
Really though, you should sit with your team mate and list the stages both of you like (neutral and CP) and pick from those based off of what the opponent's teams are. Never, EVER go to a stage either of you are uncomfortable with.
