Santander was ruled against in more ombudsman cases relating to life, pensions, and decumulation matters than any other firm in the last six months.
The bank came top of the The Financial Ombudsman list with 36% of cases going in favour of the consumer, slightly higher than and Barclays and Sesame, which came joint second at 33%.
Friends Life was next at 32% and HSBC was fifth at 30%, jointly with Aviva.
In terms of new cases brought to the ombudsman, Prudential was the highest in the life, pensions, and decumulation category at 280, ahead of Phoenix Life, 232, Aviva Life and Pensions, 212, Legal and General, 161, and Sun Life, 157.

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The latest figures also showed that in terms of investments Interactive Investor Trading Limited had the highest percentage of cases it lost.
The ombudsman ruled in favour of the consumer in 61% of cases against the firm.
Second in this list was Barclays, at 49%, followed by HSBC, 47%, TD Direct Investing (Europe) 45% and Santander was next with 43%.
Across all categories there were more cases against Lloyds (24,245) than any other firm, with Barclays (21078) second and Bank of Scotland third (19706).
The data published today details complaints received and resolved by the ombudsman service between 1 July and 31 December 2014.
The figures showed that the ombudsman took on a total of 161,649 new cases in the second half of 2014.
Banking complaints increased by 8% and investment cases by 4%.
Of the total cases referred to the ombudsman in the second half of the year, payment protection insurance made up two thirds (65%) – with a total of 104,877 new PPI complaints.
For complaints about financial products other than PPI, the total number of cases referred to the ombudsman remained at a similar level throughout the year (57,310 in the first half of the year compared to 56,771 in the second).
However, the average uphold rate (where the ombudsman found in the consumer's favour) over the six-month period was half (52%), ranging from 4% to 98% across the individual businesses.
Chief ombudsman Caroline Wayman said: "PPI complaints still make up the bulk of the ombudsman's workload and resolving these cases remains a priority. Although it's good news that complaint numbers are starting to level off, we have seen a change in the nature of the PPI complaints people are asking us to resolve – which are becoming increasingly hard-fought and more complex.
"In areas outside PPI, we continue to see many entrenched disputes that could have been avoided. We're also hearing dissatisfaction from people where their problems started with a simple misunderstanding. On these occasions, problems could often have been cleared up much earlier, if there had been better communication between the financial business and their customer.
"It will take time to rebuild people's trust and confidence in the financial sector. And a first step towards this is for all businesses to show they've dealt with their customers' complaints thoughtfully and with care. This is why we share what we see with businesses to help them understand our approach, while at the same time keeping fairness at the heart of what we do. "