It knocked last year's incumbent Macmillan Cancer Support off the top of the Charity Brand Index, put together in conjunction with PRWeek sister title Third Sector and research agency Harris Interactive.
Now in its second year, the index ranks UK charities by public perception and has been extended to a top 125.
BBC Children in Need finished at number two and the RSPCA scooped the number three slot. Last year's number one, Macmillan, was this year the number four charity.
Carolan Davidge, head of brand and PR at Cancer Research UK, said: 'We've worked hard over the last couple of years to really understand what the public think and feel about us.'