Articuno: I love the design, it has a good movepool and going through the Seafoam Cave was really fun; to this day it's still one of my favorite Ice-types),
Mew: I always found it cute and intriguing, though I've never been able to get one. :(
Ho-oh: Phoenix-based Legendary for the win! Also love the design and colors, it has a great movepool, I enjoyed going up the Tin Tower and I love its battle music.
Suicune: Its design and color scheme are pretty, and I just don't like the other two Legendary Beasts nearly as much.
Latias/Latios: They're really cute and I liked their role in the anime.
Rayquaza: It has a cool serpentine design and I really enjoyed going through the Sky Pillar.
Cresselia: It's so pretty; I love the color scheme and design. It's one of my favorite Psychic-types, though I never got to own one. :(
Shaymin: It's adorable, not to mention I love hedgehogs. I wish I could've gotten one. :(
Reshiram: It's one of the very few Dragon-types I like; it's up there as one of my favorites along with Altaria. It's really pretty and it's fun to use in battle. Also, it's about time there was a Dragon/Fire-combination.
Virizion: I love its design, it has a good movepool and I just don't like the other three Swords of Justice as much.
Victini: One of the few Event Pokemon I did manage to get (along with Celebi and Genesect). It's not terribly useful in battle and it has a bad movepool, but it's so cute and I love its animations and cry.