Hi, this is my anecdotal response amongst all the other anecdotal responses in this thread. I am currently in the latter stages of a Msc 'Sustainable Development' course at a University of London College.
I think there are a number of issues with your question. The first is what exactly you mean by 'development studies' departments. If you mean just that, then the 'best' list will be as long as the total list as there simply arn't that many independent development studies departments in the UK. If you are willing to broaden your search to include geography departments which have a recognised development geography branch, then you will be considering more options (this is exulding all the other areas concerned of/with 'development' such as politics, sociology, area studies etc). Secondly, I think you need to make a distinction between whether you are looking at a vocationally orientated course or a more academic/theoretical one (i.e. some courses are clearly designed to guide you into developmental career - say with the UN - whereas some are designed to prepare you for PhD research). The last issue is defining the 'highest ranked' department in something of the nature of 'development'. Arguably, 'development' is not like other academic disciplines and is not really an academic discipline at all. It is an endeavour, a project - its subject matter is constantely changing and so are its methods and techniques, it responds to and seeks to change the world in an every changing configuration of ways. On what terms you judge the 'success' of such an endeavour is basically impossible to come to. And if you did, it would change tomorrow.
In my experience the most important 'development' instititue in the last 30 years has been Sussex - in terms of links with political institutions, methodological approaches and theory. It has led the post-structuralist turn in general, particularly in terms of things like modern conceptions of famines, participation and participatory methods, sustainable development, rural development and livelihoods approaches.
A few people are inevitably mentionting Cambridge and Oxford in this thread. Although I have no reason to call myself an expert, they do not hold the same gravitas as such institutions as Sussex, Soas and UEA in the development field.