Short answer: it depends on what you want — but for most men in Singapore the top choices are:

• ZALORA — best all‑round, biggest local fashion selection (high‑street to mid brands), fast local delivery and free/30‑day returns options. (zalora.sg)
• Shopee — best for bargains, wide marketplace selection and frequent flash sales (use ShopeeMall/official-brand stores). (en.wikipedia.org)
• Lazada — another great bargain marketplace with a large fashion channel (now includes a Taobao Fashion channel for even more variety). (en.prnasia.com)
• MR PORTER / Farfetch / SSENSE — best for luxury / designer menswear that ships to Singapore. (apps.apple.com)
• Suitsupply (ION Orchard + online) — best if you need ready‑to‑wear tailoring / suits with in‑store fitting and alterations. (testing.suitsupply.com)
• Carousell (and recommerce players) — best for preloved / sustainable buys and rare finds. (carousell.sg)

Quick tips:

• For convenience and returns, ZALORA is the easiest one‑stop option. (zalora.sg)
• For lowest prices check Shopee/Lazada but stick to Mall/official stores and read seller reviews. (en.wikipedia.org)
• For suits or formalwear, book an in‑store fitting (Suitsupply) rather than guessing sizes online. (testing.suitsupply.com)

If you tell me your budget and style (streetwear, smart‑casual, formal, designer), I can give a tailored top‑3 shortlist.