Some would say SOAS, because of its reputation. While it has an excellent programme and is well known in the world, I think it's important to have a look at other universities (Manchester, Leeds, Exeter, Cambridge, Oxford, St Andrews, Edinburgh, Durham, Westminster) to have a feel of the degrees available. Depending on your interests, some programmes will appeal to you more than others (maybe being able to study Politics and not literature will push you towards Exeter rather than Oxford, for example).
Have a thorough look at all the programmes on offer, and don't let the reputation of the university sway your mind too much. At the end of the day, you want a degree you will enjoy. I've heard SOAS students describe Arabic Studies as "a tornado of **** that hits you real hard in the face", I've heard students at Oxford say they hated the lack of oral classes, I've heard teachers complain about how Exeter students are lazy, etc. Each university has its strengths and its weaknesses, and it's for you to decide which university would be the best for you going for the one that supposedly has the best reputation may not do you any favour when it comes to Arabic. Choose wisely.
1. The programme is very rigorous, demanding and fast-paced. Everyone takes Arabic 1 in their first year, which is three units worth of studies: three hours of classes every day (plenary, tutorials, group sessions, language lab), then regular homework and personal study. (I seem to recall being told that our first year was equivalent to the entire degree course or the first two years at Manchester.)
2. SOAS is truly international and there are lots of native Arabic speakers who will be more than happy to help you in return for proofreading their essays etc. Also, the more senior students are eager to help too. When I was in my second year, a group of final year students gave us evening classes at no cost at all.
3. SOAS is a research-intensive school. In terms of academics, SOAS really is in its own league because of the size of the department (biggest in the school), the specialisms of the faculty and the range of courses on offer. Also, there are lots of MENA-focussed courses in every other department. So you're really spoilt for choice here.
4. I was on my year abroad (at Damascus Uni) with students from Durham and Manchester. They were by far the worst students in terms of their Arabic. Just to give you a clearer picture of this: they started off in the same class as us, but after a couple of weeks they had to have their own class because their Arabic was so poor. The Oxford students were marginally better. (And I'm being objective here!) I think the reason for this difference was that so much of our first year (15+ hours a week) was spent learning the language.