Is the cheapest quote always the riskiest?
No. A low quote can come from a leaner overhead, a better method, or a contractor that genuinely wants the job. The risk is not the low price itself but what it is missing: excluded items, thin allowances for site risk, an unrealistic programme or too few supervisors. Normalise the scope first. If the gap survives a line-by-line comparison and the contractor's delivery record is good, the low quote may well be the best value.
From the RRD Field Guide, checked against the sources cited in Cheapest Quote vs Delay Risk: The Economics of Choosing a Contractor in Singapore. General information, not advice for your project.
The fuller picture
The cheapest quote is only cheaper if the job finishes on time and on scope. Overrun research shows delays are common and fat-tailed, so a small price saving can be wiped out by a few weeks of rent, lost revenue or holding-over. Compare quotes on expected total cost, including probability-weighted delay cost and the worst plausible outcome, not the tender price alone.