The Stone Roses
One might argue that the baggy scene was kicked off by the Happy Mondays given that they’d been around longer, wore the baggy clothes and were the ones who coined the term ‘Madchester’, but my recollection is that they were pretty niche prior to the arrival of the Roses (their records were only really being championed by Peel, who probably saw them as a kind of dole-scum version of The Fall).
Rather, it was overnight success of The Stone Roses debut, along with their Spike Island gig, which led directly to the explosion of bands in Manchester, and subsequently elsewhere in the UK, aping (pun intended) Ian Brown’s style. The Mondays (or more likely their manager) were quick to recognise what was happening and positioned themselves accordingly.