Short answer: it depends on the metric — but in practice two apps stand out.

• If you mean the most popular shopping app Singaporeans use to buy clothes and fashion items, Shopee is the top shopping app/most-downloaded shopping platform in Singapore. (similarweb.com)

• If you mean the leading fashion‑specific app (apps that focus on apparel/fast fashion), SHEIN frequently tops the Fashion/Shopping charts and is one of the most‑downloaded fashion apps in Singapore and globally. (apps.apple.com)

A local fashion specialist worth noting is Zalora (widely used in Singapore for apparel), though it ranks behind the global marketplaces by overall traffic/downloads. (semrush.com)

If you want, I can pull the current App Store / Google Play ranking for “Shopping” or “Fashion” in Singapore right now (downloads, active‑users or top‑grossing) and show the exact, up‑to‑date chart.