Short answer — for an all‑around, reliable one‑stop site in Singapore I recommend ZALORA (best selection of brands, Singapore shipping/returns and frequent sales). (zalora.sg)

If you want other options by need:

• Best for bargains & mass marketplace selection: Shopee or Lazada (many sellers, promos but quality/ authenticity can vary). (en.wikipedia.org)
• Best for affordable basics / high‑street staples: Uniqlo, H&M, Zara (local sites / stores with online shopping). (www2.hm.com)
• Best for luxury designer menswear: MR PORTER or FARFETCH (ship to Singapore; good for premium labels). (apps.apple.com)
• Best for suits / formal tailoring: Suitsupply (ready‑to‑wear and made‑to‑measure with a Singapore store). (testing.suitsupply.com)
• Local menswear brands / ateliers (for distinctive or bespoke pieces): Benjamin Barker, The Gentlemen’s Atelier, Q Menswear and similar Singapore labels/tailors. (gocompare.sg)

Quick shopping tips:

• Check size charts and return policies (ZALORA offers easy returns in SG). (zalora.sg)
• For international luxury buys consider shipping/GST and delivery times — some buyers use forwarding services for deals. (buyandship.com.sg)

If you tell me your budget and style (casual/streetwear, smart casual, business, luxury), I can pick the top 2–3 sites that fit you best.