For me it's Ho-Oh and Lugia, as well. I also like Kyogre, but I'm not much of a fan of Groudon, to be honest.
It's not only about the legendaries themselves, though. Looking at the cover art itself: I prefer the old art style. Not to say that I think the newer ones look bad. I just don't find them as appealing.
I understand! I liked the older art too, especially the painterly style of Ho-Oh and Lugia in the originals. The classic Kyogre and Groudon art is great too. But sometimes I'm not sure if I say that because of nostalgia or that I genuinely liked them more.
On that note, I can't forget my bias towards Unova and Zekrom <3