The best designed Legendary Pokemon from Generation 4 is Giratina Origin Forme. Don't get me wrong, the Altered Forme still looks great, but I like the Origin Forme better. It looks like a darker Pokemon, which is what I like about it. It also has some nice color selections going on, and the model of it looks great. The second best designed Legendary from Generation 4 is Cresselia. It looks magnificent and very majestic. I can't tell what it looks like, but it's pretty darn cute.
In battles, Arceus is one of my favorites to use. It's stats are so balanced and they are not bad stats either. The moves that it learns could be better, but when you teach it some TMs, it will be your best friend. It can learn a lot of TMs so that makes up for it not learning a lot of moves from levelling up. The Multitype ability can be pretty handy as well, since you can change its type when you make it hold a certain plate. Judgement, a move Arceus can learn, also changes type depending on the plate it holds which means it gets STAB.