That stunning,beautiful phoneix that caught my eye when I found it roaming in Sinnoh. I've always loved phoneixs, they just seemed beautiful and elegant to me so when I saw Moltres for the very first type,I loved it.
Also it was a unique typing Fire/Flying and could deal some serious damage and I thought it easily outpowered Ho-oh.I've also had a soft spot for the other legendary birds Articuno and Zapdos, their design was something I admired.
Other than that maybe Rayquaza. A sleek,cool-looking green dragon with weird symbols upon it's body?That is what makes an epic legendary.Also it's a big assistance if you catch it before the E4 and find yourself struggling to complete the game ^^.