I gotta go with Giratina on this one. There's something about it's design that I find really cool and unique. That gray and gold color scheme and then that red and black stripe-like pattern on it's chest is just so cool. I like that it has two forms, though I really like the basic Altered Form the most since it's the one I'm most used to seeing. I also love it's name and I like pronouncing it differently from time to time. It's a fun word.
Altered Forme Giratina, without hesitation. Everything from its appearance to its typing to its versatility in the Ubers environment is just epic. I love the six legs, its towering, imposing presence and its stocky build. My Giratina plush is definitely among my favorite in my collection, too. Even its name is fun to say.
Out of all of the 4th gen. legendaries, my favorite would have to be Giratina. The desgins and color scheme of both Giratina's Altered and Origin form are just really cool to me, although I do prefer the Origin form over the other form. Giratina's unusual typing of ghost and dragon also appeals to me, mainly due to the fact that I like unusual type combinations. Plus, Giratina is one of my favorite legends to use in battle.
;483; Diagla's pretty awesome with time control and being a dragon, but I'd say my favorite 4th gen legendary is Uxie. ;480; Uxie reminds me of Mew, only grey and with yellow bowl-cut hair, two tails, and a sort of blank sleepy look... I think that's awesome: just looks so neutral and chillaxed. I called my Uxie Gnosis (greek for knowledge) cause he's supposed to be the knowledge pokemon.
Based off appearance I would have to go with Origin Forme Giratina, as many of you have already pointed out he just looks awesome, however, if based on my favorite to use in battles I would have to go with Azelf, it has served me well on many occasions and I cannot deny how much I like it.
Darkrai is my favourite, followed closely by Palkia, i'm not sure why to be honest. I guess because Darkrai was my first Event lengendary and Palkia was my first proper legendary in the fourth gen (my brother traded me the 'aliens' just so i could get him)