Shaymin, hands down. Regular is already fairly okay, but Sky Form is nothing short a complete beast. STAB 120-base power move with 80% chance of dropping SDef two stages? Stab Air Slash (with 60% flinch rate)? Look no further. Even regular Shaymin can pull off a decent special hammering with Seed Flare, and gets some other useful options. Not stellar; but far better than gigas.
Regigigas, on the other hand, is a big, stinking can of phail if ever there was one, simply because he has a trait even makes Truant look okay in comparison. There is simply no way you can keep that thing in for five turns against anyone competent without having it run over. It's just. Too. Slow. The only option is running some kind of lukewarm special attack set, but there are lots of other pokémon who do that far better already. D:
So yeah, unless you're in double battle and can get rid of that god-awful trait with Trouble Seed, Gastroacid, or Skill Swap then you might as well forget about Regigigas. :<