I can't say I'm a huge fan of legendary Pokémon, but those that I like, I like alot. I'm not completely sure which one is my absolute favourite, but the legendaries I like are:
Articuno, Zapdos, Raikou, Mew, (only the anime and manga Mew. Its game sprites suck and it's pretty shabby for an uber. =| ) Lugia, Latias, Groudon, (to a certain extent) Mewtwo (once again, only the anime and manga Mewtwo. For some reason I feel sorry for the anime Mewtwo and the manga Mewtwo has a too awesome fighting style to be ignored. Giant spoon ftw! =D) Yukishii, Diaruga and Kureseria (due to its utterly random and freaky look. O.O). Out of those, the only Pokémon that are on my favorite Pokémon list are Articuno, Raikou, Zapdos, Yukishii and Diaruga. Now let's round those Pokémon to three.
I have always liked Articuno and it's classic so Articuno'll stay for my top 3 favorite legendary list. I have loved Yukishii ever since I saw it. I dislike Psychic types in general, but something about Yukishii makes me want to train one. For the third one I shall pick Diaruga due to its utterly cool typing. I have wanted a Dragon/Steel type Pokémon ever since G/S/C and now we finally have one! ^o^ Those Pokémon in order from favorite to least favorite would be:
1. Yukishii (one of the few Pokémon that's close in awesomeness to even Dunsparce, Feebas, Cradily and Aipom. ^_^)
2. Articuno
3. Diaruga
I'm not sure if Lucario is legendary.
If it is: Lucario
If not: Ho-oh
(300th post)
Just to confirm your doubts, Lucario isn't a legendary.