The annual survey by Which? found cosmetics firm Lush was the "clear winner" according to consumers, the only brand to achieve an "excellent" five-star rating for having knowledgeable and helpful staff and for making customers feel valued.
But the watchdog suggested Ryanair, TalkTalk and Npower "pull up their socks" after they limped into the bottom three places of 100 big-name brands.
The survey rated how the UK's biggest brands measured up on customer service alone, finding that friendly, helpful, knowledgeable staff and a quick resolution of problems were "very influential" for consumers when choosing a company.
An accompanying poll found 81% of respondents said it was important for them to feel like a valued customer and 86% would leave a brand that treated them poorly.
The survey found Ryanair, TalkTalk, Npower, 99p Stores, TK Maxx, BT and EDF Energy all failed in their treatment of customers, "but BT, TalkTalk and Ryanair deserve special mention for 'poor' two-star ratings in every element of customer service provision", Which? said.
More than a third (38%) said they were irritated by being made to wait when they needed help or advice.
:: Which? surveyed 3,331 members of the UK public online in July.
A Which? spokeswoman said: "While many companies performed well, it's disappointing that some of the biggest household names are still failing to meet consumers' expectations of good customer service.
"Nearly nine in 10 people said they would leave a brand that treated them poorly so it's clear there can be no room for complacency. Some companies will have to do far more than compete on brand awareness or price alone if they are to retain their customers in these times of financial squeeze."