Ask who writes the code and where they sit, whether design and development are the same team, and what you receive at handover. A website development agency that cannot hand over a repository and deployment configuration is selling you a rental, not a build. Ask how performance is measured and what the number is before launch, not after.
Directory lists of the top web development companies are ranked by who pays to be listed at least as often as by output, so treat them as a starting shortlist and nothing more. The best web developer for your project is the one whose recent work looks like the problem you actually have: a catalogue, a portal, a booking flow, a documentation-heavy product site. Ask to see a build of that shape and ask what it cost.
There is no single best website design and development company for every brief, and any shortlist that claims otherwise is selling a ranking rather than a fit. Judge a website design and development agency on three practical things: whether the same people handle design and build, whether the quote separates the one-off build from ongoing care, and whether they will show you a site of similar complexity and explain the decisions behind it. Those answers tell you more than an awards page ever will.
Ask how the quote changes when scope changes, and get the page count in writing. Most disputes on web design and development projects are scope disputes wearing a technical costume. Ask what happens to the URLs you already rank for, because a rebuild without a redirect map is the most common way a new site loses traffic in its first month.
Where we are the wrong fit, we will say so early. If you need on-site workshops in Singapore, a locally registered vendor for grant support, or a very large in-house-style team seconded for months, a Singapore-registered firm is a better answer than a remote studio, and choosing one is not a loss for you.