4. Boyz II Men - II - 1994I don't know that "surprisingly" works here -- I'm partial to their debut, because I'd be satisfied with basically anything so long as it had those four or five ubiquitous New Jack sound effects in it -- but II is probably the most successful transition any boy band has ever made to young-adult-band, in terms of simultaneous critical and commercial approval. In hindsight, there was nothing at all guaranteeing that the playbook behind "Motownphilly" would work as they slid more fully toward the II Men side of the conceit.
Of course, 2000's Nathan Michael Shawn Wanya, their true We're-Real-Artists-Man album, failed to connect with audiences, which probably serves as a terrifying warning to all boy bands navigating this transition. These are treacherous waters.