The last proper roaming Pokemon (i.e. you find them at random and you have one turn to catch them before they flee and they keep doing it forever) were Tornadus and Thundurus, in Black and White respectively.
X and Y had Articuno, Zapdos and Moltres as kind-of roamers, but the mechanics were quite a bit different (read: less annoying). They would immediately flee when found, and would flee from 11 encounters before retreating to the Sea Spirit’s Den, where they would STOP roaming. So whether you’d even count them as roamers is up to you, I guess.