Berenberg has named Asos, Next and Supergroup as the "long-term winners" among UK fashion retailers, maintaining a 'buy' rating on all three stocks.In a review of the sector, the broker highlighted that competition online for clothing retailers has increased, but stressed that a strong presence on the web will not be enough in itself to drive outperformance."We analyse the product choice, newness and exclusivity, market-positioning and proven international growth potential of each retailer to determine whether there is a sustainable competitive advantage," it said.As for Asos, Berenberg believes its focus on the twenty-something niche market provides a "sustainable competitive advantage" and lower customer acquisition costs.Next, meanwhile, continues to be ranked in the UK top 10 brands by WPP "and we believe its clear and consistent promotional policy attracts both customers and third party brands".As for Supergroup, the broker reckons the company can benefit from an increased focus on product and is beginning to demonstrate its international growth potential.Asos was trading up 1.3% in morning trade at 3,858p, Next gained 0.1% to 7,387.5p while Supergroup fell 0.3% to 1,158p.