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 wanted 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.
The poll also found almost half of customers were irritated by staff having private conversations, often about "non-work related issues", and 50% were annoyed by excessive queuing.
More than a third (38%) said they were irritated by being made to wait when they needed help or advice.