And although Waitrose shoppers complimented the upmarket chain's "top quality produce" and helpful staff, overall customers did not believe the supermarket offered good value for money.
A Sainsbury's spokeswoman said it was "confused by the methodology" used to create the rankings, pointing out that its stores had outperformed the top-ranked in-store supermarket in the star ratings system on measures such as store appearance, queuing time, staff availability and range of products and had matched it on quality of own-label products and fresh products. The chains are scored on customer satisfaction and whether they would recommend the store to a friend.
The larger supermarkets did worse off in the survey, with Sainsbury's (LON: SBRY) coming last of the nine.
"With food costs rising it seems as though shoppers have felt the pinch and are voting with their feet and wallets".More news: Sport an afterthought as Unified women's team takes the ice
This was largely down to a two-star rating Waitrose was given for value for money, compared to Aldi's five.
The news comes as Tesco continues to feel pressure from Aldi and Lidl, with the latest Kantar data showing earlier this month that the blue-chip's grocer market share had continued to decline in the 12 weeks to January 28, having dipped 0.3 percentage points to 27.8 percent.
Iceland managing director Nigel Broadhurst said: "We are naturally delighted to have been recognised once again as Britain's best online store in the annual Which? supermarket survey".More news: Russian figure skaters win Olympics silver in team competition
"The big four supermarkets need to up their game or risk losing their customers to other supermarkets who are doing a better job of giving people what they want", said Neill.
In the online category, Iceland came joint first with Ocado, marking the third consecutive year the frozen food specialist has trumped its rivals.
Amazon Fresh, which only launched in 2016 and is only available in the south east of England, came in fifth position.More news: White House: Trump 'shocked' by domestic abuse claims against ex-aides
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