Legendary pokemon. The most powerful, the rarest and the most sought after pokemon in the game. But does being legendary make them amazing. I'm not sure. I tend to ignore legendary pokemon because most are just hyper beasts with little likeability. One legendary however is very cool and powerful without being tedious or just crazy. Dialga.
Gen IV introduced some pretty poor legendaries, including some I don't really count as legendaries. Dialga, however, is cool, powerful-looking and very likeable. There's very little to explain or talk about. You can just see its power and brilliance when it appears in Pokemon Diamond or on Super Smash Bros. Brawl. This is one huge beast thats not to be messed with.
Dialga, like most legendaries has a counterpart, in the form of Palkia. Palkia is cool and all, but its a bipedal dragon, we saw one of them six pokemon into Gen I. Dialga is abstract without being silly, powerful without being uber and a beast without being ugly.