Which Regi should i choose for my doubles wall?

Regice w/icy rock
Bold
EVs 252 DF 252 HP
Blizzard
Hail
Ancient power
Thunderbolt/Charge beam
Hail gives blizzard 100% Acc

Regirock w/smooth rock
Bold
EVs 252 SD 252 HP
Rockslide
Sandstorm
Ice punch
Thunder punch
Sand boosts Regirocks SD by 50%

Registeel w/leftovers
Adamant
EVs 128 DF 128 SD 252 HP
Iron Head
protect
hammer arm
shadow claw

Regigigas w/ leftovers
Adamant
EVs 128 DF 128 SD 252 HP
Revenge/Drain punch
Return/Dizzy punch
Rock slide/Fire punch/Forsight
Protect

I like Regirock, but I might suggest replacing thunder punch with earthquake, as it can do damage against both opponents in a doubles battle.

I say registeel because he has a resistance to dragon and is immune to poison what more do you need. Plus HE ROCKS.

Hmmm... I wonder what that is supposed to mean...!

You can't go wrong with any of them, since they all are freakin' awesome, but then again that's a biased opinion since the Regis are my favorite Pokemon. (minus Mr. Gigiggigigigigas)

But... in Doubles, you have to be more versatile and flexible due to the large variety of strategies that can be implemented. Thus, my vote goes to Registeel since it can be more versatile than the other two by default. Although, I'm no expert on the metagame, so I suggest looking for some other opinions first before deciding.

Also... might I suggest making three teams with each team having one Regi?

Why does everyone hate Mr. Gigas?

Anyways, Registeel works well thanks to typing, and he's more well-rounded than the others. However, if you want a team solely on Defense or Spec. Defense, use Regirock or Regice, respectively. however, Registeel works the best. Regice is good for hail teams, too.

It's slow start ability, But there are some quite nasty parafuse/flinch sets Sir Gigas can do quite well.

I'd go with Regirock if you could use it with sandstream support so it could get leftovers and EQ. Same with regice. Registeel is good, but usually works best as toxi-tank or rest-talker.

Slow Start, Normal-type, and it's ugly. How can they add a pokemon to a 3rd-Generation legendary trio, and it's not even a similar type? It shouldn't even exist. The only good thing about it is that it unlocks the other Regis in Platinum. Btw, I have 2 Event Gigas's if anyone wants to borrow one.

what about Lugia and Ho - oh Compliments to the trio of birds in the first gen
And being Normal it only has 1 weakness fighting which ALL Regis are weak to and only three types are resistant to normal attacks
besides while slow start is in effect it is very simillar to registeel
Registeel-Regigigas
HP 80 - - 110
AT 75 - - 80 160
DF 150 - 110
SA 75 - - 80
SD 150 - 110
SP 50 - - 50 100
Slow Start effect

Thus using that logic I'm throwing Regigigas
into the mix

Regigigas w/ leftovers
Adamant
EVs 128 DF 128 SD 252 HP
Revenge/Drain punch
Return/Dizzy punch
Rock slide/Fire punch/Forsight
Protect

That's not how it works. Regigigas' actual stats are halved, not its base stats; its base 160 Attack means that it has a maximum stat of 460, and 460 is halved to give Regigigas a max Attack of 230 under the effects of Slow Start. Regigigas basically has a base Attack of 55, because base 55 stats have a maximum of 230. And his Speed is basically base 25 (slower than Magcargo).