Rayquaza got 438 attack power to work with. Keep that up with dragon dance will result in a deadly beast. Having extremespeed doesn't allow other team's quick attackers to take out Rayquaza too after it's been weakened when it's dragon dancing.
Kyogre got a natural rain dance to boost its again, over 400 sp.atk. It got free thunder to use, a water attack that's boosted by STAB already, along with having ice beam. Having no physical weakness doesn't help either. Auto-permanent rain dance allows a whole team of rain dancers to follow up and sweep the rest of the party as well. Having the last attack as HP Grass results in complete destruction. Even Blissey will have a terribly hard time against water spout with such high sp.atk, and rain dance/STAB boost, which is a proof of just how strong Kyogre is. In my opinion, one of the strongest of the ubars, along the calmkou rank.
Registeel isn't even an ubar, and is at most winced about. Its usefulness is only limited, and is one of the least useful legendaries, only above Articuno, Moltres, Jirachi and possibly Entei/Regirock as well. Its most problematic thing is that its movepool is in the middle of nowhere, and how it's usefulness is overshadowed by Regice the infamous boltbeamer.