One of the most frequently asked questions. Entirely fair but really difficult to answer. There are so many variables, some of them personal to you. Ask staff in your own department.
Academic links are really strong and they'll know exactly where the hot spots are.
Or check the Research Assessment Exercise of the Joint Funding Councils of the UK which produced research ratings for individual departments nationwide. Although the last such exercise dates back to 1996 a new one is being undertaken this year. If employment prospects are important to you don't be afraid to ask what their students go on to do. The PhD's relationship with their supervisor is really close. Is this someone who you can respect and get on with for three years.