Redbridge Borough Council received the glowing report from inspectors who gave the department its highest rating of "outstanding" overall, following a previously "good" judgment three years ago.
The only category that remained good was the experience of children in care and care leavers which, despite investment, remains "variable", largely due to staff changes at an external provider.
Hampshire and Lincolnshire county councils were also rated among the top providers in the country, having progressed from "good" to "outstanding" since they were last inspected nearly five years ago.
Inspectors, who visited between April and May, said strong leadership was a key factor that had contributed to all three councils' improved ratings.
For the county councils, targeted investment was also an important driver.
Hampshire's report, which rated the county "outstanding" across all categories, states that "children's lives consistently improve as a result of the help they receive".
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Director of children's services Steve Crocker and his leadership team have "resolutely focused on continuing to improve the help, care and protection provided to children," the report states.
Social workers were singled out for praise for being "highly skilled at building meaningful relationships with children, engaging them in their assessments and plans".
"Children in care benefit from high-quality support, which is making a real difference to their outcomes.
"Management oversight of practice is clear and considered, ensuring that children's plans progress at pace," the report concludes.
Inspectors flagged the council's "strong political and corporate support" coupled with "well-targeted financial investment" which had helped its leadership team to implement an "ambitious" transformation programme.
"This has created the capacity, training and infrastructure to enable social workers to engage more purposefully with children and their families," the report says.
Hampshire County Council leader Keith Mans said: "We have always been proud of the high standard of children's services delivered in Hampshire and of our reputation as one of the highest performing authorities in this area of work - evident in the many partnerships that we have with other councils who have sought our guidance and expertise to help further improve their own services."
Inspectors also highlighted Lincolnshire's "highly skilled workforce" and its "outstanding" service for children in care.
"Permanence and stability are achieved quickly for children who cannot live with their birth parents and there are high levels of multi-agency support to help them reach their full potential," the report says.
The council had responded well to the findings of a focused visit in 2014 and implemented a new quality assurance framework which had delivered improvements to services for children, it adds.
"An insightful, highly motivated and extremely child-centred senior leadership team has been relentless in its quest to improve the life experiences of and chances for children in the county," the report concludes.
Inspectors rated the council as outstanding across all areas except for the experiences and progress of children who need help and protection, which retained its previous "good" rating.
"We are officially rated one of the best children's services in the country, which is a tremendous accolade and just reward for the commitment of staff, partners and all those children and families who work with us to make our support so effective", said Patricia Bradwell, executive councillor for children's services.
"We support some of the most vulnerable young people in the county as a key priority and this proves we are doing the right things at the right time to ensure our services for children are the best they can be," she added.