Don't worry, I'm not supporting Ground this time.
Well, I'm going to give arguments on all the types given so far:
Normal:
Normal lacks weaknessess, yes, which makes it almost never take x2 damage. However, it is devoid of resistances, except an immunity to Ghost [Why is Normal Immune to Ghost anyway?]. In addittion, it's lone weakness is Fighting... which is a moderately common attackinng type. Finally, Normal STAB is not the best in the world, especially with all the Steels and Ghosts running around.
Bottom Line:
I feel Normal won't get the job done
Poision:
Poision, I am dead set against. Yes, we're only weak to two types. One of them is Ground the single most common attack, Earthquake, being this type. That alone is enough reason to avoid Poision, unless we're planning on Posion/Flying, or a levitateing poision. In addittion, resistance-wise, I'd say Poision is WORSE than Normal. How often do you see poision-attacks in OU? And how often do you see grass-type attacks? A lot less than Ghost.
Oh, and let's not forget the 2nd worst STAB in the game.
Poision basically nessessitates we have Levitate or Flying typing, and even then, is probobly not going to threaten
Water:
Water, is actually a decent option. Thunderbolts are common, but, we've all seen bulky waters [Bearing in mind, that's the route our concept assessment headed], like Suicune shrug them off. Grass-type attacks, again, are uncommon. Water actually has a useful STAB as well, and basically comes pre-packaged with Ice Beam, allowing a Water CAP 10 to hit most of the Top 10... that's why we chose it for Argonaught, after all.
Same number of weaknesses as Poison, better resists, and better STAB? Speaks for itself.
Water is a decent option, and should be considered
Flying:
SR weak. That alone should end the discussion, as counters don't want to be switching in at 75%... and then whatever they take from switching in. Flying is also a poor STAB in OU. Weakness to Ice, Electric, and Rock are also crippling... although, the Ground Immunity and Fighting Resist are tempting. However, if Salamance didn;t have the Flying type, so took half damage from SR, it might just find itself in Ubers [Intimidate weakens the EQ's, and you lose Ice Beam weakness, Stone Edge weakness, T-Bold neutral...]
Poor STAB and SR weakness make Flying a poor option
Ghost:
First thing that springs to mind in Persuit Bait. Bearing in mind that whatever this AP is countering may full-well switch out, meaning CAP10 will need to switch out, to counter it later, we do NOT want Persuit bait.
Ghost STAB is also below average, with Steels, and Darks resisting it, as well as not getting that many Super Effective hits. Fighting and Normal Immunities are nice.
Risk of beng Persuit bait makes Ghost a poor option
Steel:
Tons of resistances to switch in on. However, a lot of these are to rare attacking types [Kinda screams about the Steel-domination, eh?]. In addittion, Steel STAB is bad, it's not going to threaten Gyarados, who'll just set up in your face, or Heatran, who'll just Fire Blast you. Then there's Earthquakes. [And Earth Powers]
Poor STAB and common weaknesses makes Steel an average choice at best
Fighting:
Nice STAB, lack of common weaknesses, we've all seen what Machamp can do, and he's one of the few mono types in OU for a reason. However, Ghost types are the bane of this idea... and there's also Flying types like Mance and Gyarados.
Fighting is a decent type, and is right below Water in my opinion
Right, so, with the so far suggested ideas, my support is with Water. However, I would like to also bring in another thing to the equation.
Grass
I know what you are thinking. Grass STAB is quite poor, but, listen. One way of countering is to kill something outright. Another way is to cripple something so it can be stopped easier, by anything. Whhat type gets more status moves than Grass? Grass also has a few key resists, to ground and water, but, unfortunetly, is weak against Fire and Ice.
In addittion, Grass types do have the potential to hit hard. Leaf Storm is basically Draco Meteor, after all. Heatran coming to sponge it? Thunder Wave/Stun Spore.
What the pokemon counters would fully depend on it's set. A grass CAP10 would be helpless against Salamance unless it carried a Paralysis move, which would cripple Mance to the point of uselessness, however, it would probobly faill against Celebi, unless it had Toxic. 4MSS is going to keep the countering in check, on any pokemon.
Finally, what's the general thing that happens when a counter comes in? You switch out. So, status moves would actually give CAP 10 some use rather than 'I switch in, they switch out, I switch out.', it would be 'I switch in, now do they want to sacrifice something to status?'