I'd say probably Suicune, maybe? Or Ho-Oh. I doubt I'd have caught the roaming Pokemon straight away, and I remember I was trying to catch Lugia the day I got R/B/Y, and I was getting my hair plaited when I got Ho-Oh which I'm 99% sure was before that, so yeah I'd have to go with Suicune due to not being able to remember when I caught it, thus assuming it was first. ;x If not, then Ho-Oh, since I kinda can't remember the events of Gold aka which comes first, lol.