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Before I start, I want to point out that pretty much everything here is gonna be my opinion. I won't be writing IMO, though, as it will just clutter everything up.

While Generation IV did a lot of things much better than the three generations before it, there was one point where it sucked majorly: the legendaries. With a couple of exceptions, they all have terrible designs. I'm gonna be talking about each duo or trio of them one by one to tell you why I say that.

Uxie, Azelf, and Mesprit


The Lake Trio has one major flaw: it's three times the same pokémon. They are all blue flying pixies with a double tail and a weird hair-thingie on their head which is a different colour for each of them and they're all Psychic type. The legendary trios of the previous generation were each unique in a very distinct manner, which each being a different type and a different, though still similar design. The only part of that that the lake guardians have is the similar design. They're supposed to represent sadness, happiness, and willpower or something like that, but I'll be damned if I can remember which emotion goes with which colour hair, or which colour hair goes with which name, or which name goes with which lake, and I played Platinum more times front to back than I can remember.
My question is basically: why not make them three forms of a single species? Or better yet, why not actually bother designing three distinctive pokémon?

Dialga, Palkia, and Giratina

Here we get to the worst designed bunch of them. IMO Dialga and Palkia are the ugliest of all 493 pokémon. There's also the problem of these three being called the Dragon Trio though none of them looks remotely like a dragon.
Let me start with Dialga. It looks like a big, awfully ugly blue dog with some metal crap attached to it in random places and a skull like those aliens in Kingdom of the Crystal Skull. It doesn't even look remotely like a Dragon, and it's cool Steel/Dragon typing is unfortunately wasted on this piece of crap.
Next up is Palkia. With it's tail, long neck, and wing-like thingies, this one looks a tiny bit like a dragon unlike the other two. But it looks way more like some sort of armoured soldier with huge toes wearing stripy pants and standing in a wrestler's pose. Fighting-type would be much more approppriate than Dragon-type. It also is pink. Why the crap did they make a legendary who was (probably) supposed to look cool and badass pink? (Before you interrupt me, Mewtwo is purple and mew is supposed to be cute, not cool).
Now we get to Giratina. Contrary to what you may expect, I actually like Giratina's design. It's a gigantic badass centipede, and that's a really cool idea. There's also the fact that a legendary Bug-type has never been done and- What? It's a Dragon-type? How in the name of Davy Jones' closet organiser is it a dragon? It's a centipede! While I really like it's design, I get really annoyed by it's typing.
Another thing about these three that really annoys me is what they represent: Time, Space, and Antimatter. Firstly, what the crap does Antimatter have to do with the other two things? And why did they even make a legendary to represent it? There is hardly any antimatter in the universe, because all of it got annihilated about a second after the Big Bang. And then there's the fact that having the creatures that created the universe catchable is just plain ridiculous, but I'll talk about that when I get to Arceus.

Heatran

Heatran stands out among the generation IV legendaries as an actual decent design. The only complaint I have about it is that it's volcanic theme is a bit of a "been there, done that" thing after Entei and Groudon.

Regigigas

Meh. That's really the most descriptive thing I can say about it. The Regi's always were rather boring legendaries (especially their incredibly original names), and they didn't really need a boring trio master with a detrimental ability. I hope I just used the word detrimental right, because to be honest I don't know what it means.

Cresselia and Darkrai

I really like these two. They have surprisingly good designs, especially Darkrai. I like their themes, their typings are fitting (though Psychic-type legendaries have been done a lot before). No complaints about these two.

Manaphy and Phione

The problem with Manaphy is that it's just this generation's Mew-ripoff. We've already had forest-Mew and space-Mew, so now we have ocean-Mew. While the fact that it's able to breed into Phione should be a pretty cool and destinctifying thing for it to do, it really isn't, as Phione is pretty much the exact same pokémon as Manaphy except weaker. It looks very similar, has the same, but lower base stats, and the same moves with two exceptions.

Shaymin

Here's a hedgehog who can turn into a flying donkey.
That's pretty much all I have to say about it. It's not interesting enough for more. I guess at least it's not ugly as hell like the UnDragon trio.

Arceus

Arceus is quite possibly the silliest pokémon of them all. It's the creator of the universe, the pokémon world's equivalent to deities from all the various mythologies, and it's a llama with a ring around it's middle. Ooh! It can change colour! And eleven-year ld kids (Though I believe the pokémon planet has longer years than Earth, but that's a matter for a different time) can catch it and use it in a competitive sport. What the crap?

Well, that concludes my little rant. Hope generation V will get better legendaries.