For me, it's a 5-way tie between Darkrai, Arceus, Dialga, Girantina Origin Forme, and Shaymin Sky Forme. I've rather disliked most legendaries, but the 4th Gen has some cooler legendaries than other gens.
I have a tie-way as well! Mine's are:
1. Giratina! It's Origin Forme is much better than it's altered forme. It looks cool, it's Shadow Force is devastating, and it has pretty awesome battle music =D
2. Palkia. It was the first 4th generation Pokemon I caught when I got Pearl, and I always thought it looked cool, and it's attacks are beast. And, it's the creator of space!
3. Dialga. Another cool legendary which is also really powerful, and the creator of time. Roar of Time has to be one of the best Pokemon moves ever =)
4. Darkrai, the King of Nightmares. It's a very unique and frightening Pokemon that I'd probably prefer over Haunter. If it puts you to sleep, it's over!
5. Shaymin Sky Forme. It looks way different from it's original forme and I prefer this forme over the original. I also liked the way it was heroic-like in the movie xD
6. Arceus.. Probably the "ultimate" legendary Pokemon, seeing as how it is the creator of the Pokemon universe, and it's base stats are at a whopping 720, which makes it very powerful, and has the ability to change types. It's battle music is pretty cool too.