Shaymin, by far. Slow Start just hurts Regigigas too much, and Shaymin may have a shallow move pool at the moment, but we also have Sky Forme coming that could help with that (and 80% chance of halving Special Defense with Seed Flare is just too attractive to ignore IMO).
and they just killed legendaries when they put in 13 of them in the fourth season.
Yes, but like it was said before, with slow start, it requires strategy.
Regigigas is just one of those pokémon that you can only rely on with brute strength.
Also don't forget that regigigas has a base defense of 110, so it will last a while against most pokémon.when its killed, you have permission to call it regicide.
shaymin has seed flare, and thats about all its worth.
Celebi and mew are stronger than shaymin, and have better movepools.
In my opinion, shaymin is the weakest legendary, and they just killed legendaries when they put in 13 of them in the fourth season.
No, with Slow Start, it requires waiting for five turns while the opponent gets to set up enough that the high Attack won't matter. Besides, beefing up its defenses means sacrificing some of that Attack stat you're waiting for.
Mew is Uber right now and Celebi may be more useful in several situations, but that's only one superior compared to the several Legendaries superior to Regigigas.
Actually, I thought the consensus was that either Articuno or Entei were the weakest.