Oh man we talking legendaries now?
Alright I kinda sorta hate these things
Now, in terms of battling in Ubers, I love these guys for the most part. Arceus is very interesting, Giratina's a great wall, Palkia's a fantastic special attacker and Dialga... well, I like it's typing I guess.
But in-universe and story wise, these guys went a little too far, and to some extent the lake trio. I know this has been said so many times and it's almost cliche by now, but at the point where you can stuff God into a Poké Ball, along with the concepts of Time, Space, and Antimatter, it just feels a little too ridiculous. I much prefer the legends of BW and XY story-wise, because while they obviously command a lot of power and there's a lot of lore surrounding them to make them deserve the title of Legendary, they are self-contained stories confined to their regions that are not gods.
Gen 4 they just went fuck it. Let's make gods. This, to me, is not only ridiculously overpowered, but it means that storywise no other legend can ever surpass or even be on the same level as these things - and thus diminishes the 'legendary' status -, and in addition it makes these Pokémon aside from Giratina honestly very boring. There's no real room for development because they're absolute. Dialga is Time. Palkia is Space. Arceus is God. That's it.