It's a two way between Tyranitar and Hydreigon. For both: They're both based off of two extremely popular giant monsters(moreso in Tyranitar's case though)
For Tyranitar: I loved his debut in that Celebi movie. After that he just became a must have for me. During the Gen 3 days I was really obssessed with catching a Larvitar.
Hydreigon: When I was skimming the leaked 5th Gen roster on Serrebi, it immediatley caught my eye. I just love the fact that it's a 3-headed dark type hydra-dragon. Truth be told, one of my favorite design aspect about Hydreigon...is it's eyes(main head). I dunno, I just love the black eyes with no pupils, except for those red marks.
For Legendary, Yveltal is currently my all-time favorite atm
Official hot fudge sundae of the IDF Official Hydreigon of the Pokemon B/W 2 Boards.