As for the weakest legendary, I'd say Regigas. I think Slow Start + the pure normal typing made it feel rather weak in comparison to say, the other Regis or the three birds. :(
I was actually contemplating answering with Regigigas as well, but it's only really weak from a competitive standpoint. It gets hugely strong if you're able to stall long enough for the effects of Slow Start to wear off, and that's typically not too hard when playing against the AI, which was why I opted out of not going with Regigigas. Typically I don't find Normal typing to make a Pokemon that bad either, since they only have one weakness, and can typically learn a variety of different typed moves. The biggest downside is of course that they don't get STAB, but I've been able to live aroudn that haha
Which legendary do you think has the most power over the universe, after Arceus?
I think the power displayed by Mewtwo in the first movie alongside the introduction of its two mega forms in X/Y makes it one of the strongest legendaries in the pokeverse. Though I feel like based off the events of D/P/Pt and ORAS that the creation/weather trio could both be up there too. It's tough to say, really.
Well here goes!
Gen I: Mewtwo
Feel like there's not much to expand on this one. The bird trio are strong as a single unit though as individual pokemon, they're rather eh compared to other legendaries. I think Mew could definitely contest Mewtwo though based on their fight in the first movie, they seem to be on pretty equal footing. Gonna go with Mewtwo for the sake of the tougher looking design!
Gen II: Lugia
In a straight up fight, I'd say Lugia would have the advantage against Ho-Oh. Mainly because we see more evidence of its power than Ho-Oh with Pokemon 2000 and the fact that a 'guardian of the sea' would more likely douse fire than get destroyed by it. As mentioned previously though, I don't think it makes sense for it to be a psychic/flying over water/flying. Guardian of the sea, making it water to complement Ho-Oh's fire/flying type, what am I missing here? Did they make it psychic so it could sound like Morgan Freeman when telepathing with Ash and co or what?
Gen III: Rayquaza
RAUUURRUAR
Gen IV: Giratina
As ringleader of the Creation Trio, I'd imagine that it's the strongest out of the lot. I think the whole Distortion World idea's quite spooky too and adds a whole other layer to Giratina. Though I feel like Dialga's title as 'ruler of time' is probably the best out of all them so I dunno. I'm going with Giratina just because it's the leader though.
Gen V: ???
I did play both B/W and BW2 though I have no idea on the power of any of the legendaries. I really liked the Four Swords pokemon though - the concept and designs behind them were all fascinating.
Gen VI: Yveltal
Though Yveltal would be beaten by Xerneas due to type advantage, I feel like its the more menacing and intimidating legendary of the two. I'd comment on Zygarde though I know nothing about it so for now I'm gonna go with the huge flying y!
How do you feel about Giratina? Seeing as its the ringleader of the three, I'd imagine it to be stronger or on the same terms of Dialga/Palkia. Though after watching the clash between them in that one movie, I can see why you'd think that.
I agree with pretty much all of these except for Lugia and Giratina. For Lugia, I think just the fact that it isn't actually a Water type is enough to argue for it not having an edge on Ho-oh. I mean, even if it is the guardian of the sea, if it's not represented in any way on the Pokemon other than the location you found it in, I hardly think it would matter. Ho-oh on the other has a backstory that kinda makes it seem more powerful. Apparently it only shows itself to certain kind hearted people or something, and it created the legendary beasts by reviving dead Pokemon. And to add onto that, it's given its own shrine basically, so that would argue for Ho-oh being the one that the inhabitants of Johto worship the most. Though as I mentioned earlier, I believe Lugia used to reside at the top of the burned tower, so it might just be due unfortunate circumstances that it doesn't seem to get the same praise. In any case, just by going from a lore point of view, Ho-oh should be the most powerful.
And as for Giratina; I think all three from the creation trio are equally powerful honestly. Even though Giratina looks like it's stronger in the movies, I think that's simply because it managed to drag Dialga into its own world, where it would make sense for Giratina to be more powerful. I honestly don't think there's a whole lot to say to this, other than they all seem to be given equal power, and were just created for different purposes.