You can't answer the question o'whats a top university' unless you expand the question to 'top for what?'
In world rankings - only Oxford, Cambridge and imperial make the top 20. UCL and. LSE come next.
For research rankings - measured in a massive national exercise every 5 or 7 years - you can get a ranked list both overall and for each subject group. So you can be top for one subject but in the middle for others whilst being either near the top of the overall list or near the bottom of the overall list.
Or you can be top for graduate destinations - ie have the highest percentage of students in employment after 6 months or 18 months.
Or you can be top for educating the highest number or best prepared vocational graduates - so health professionals and teachers.
You just can't answer the question without more specific criteria.
But in general, the RG unis which appear on most of the lists for most aspects near the top of most of them are the usual suspects - Oxford, Cambridge, imperial, UCL, LSE, Edinburgh, Durham, Bristol. Noting that Durham didn't even join the RG until 2012 so the grouping really doesn't imply much.