yep im doin law at merton.
in terms of "prestige" i would say magdalen is probably the best college for law, a hell of a lot of law lords went there and its generally considered excellent for law. however brasenose has a reputation for being "the law college" and from what i hear it has a very strong tradition in law.
if you're looking at "results" then the best place is probably merton, and i would recommend merton as the best overall (not that im biased!!) Merton lawyers always get at least a 2.1 and has the highest number of firsts as well, the lawyer community is excellent. lawyers are very cliquely (usually because we work harder than the others) and you become very good friends with your fellow lawyers.
basically all colleges offer law and the best ones are going to be the ones near the top of the norringtion (of course this assumes you are looking for the most prestigious/respected) such as Merton, Magdalen, Christ Church, Balliol, St John's. The advantage of being at the top colleges are:
1. You are more likely to get a 1st/high 2.1 (a 2.1 is essential if you plan to go into a career in law, employers demand a 2.1)
2. Colleges exchange tutors for certain subjects, being at a top college ensures you will always get the tutors who are most respected within the university.
But studying law at any Oxford college is great, just go for the one you like the best.
I'm not too sure bout en-suite facilites, at Merton you are very very unlikely to get ensuite in your first year unless you have a special condition e.g. diabetes that means you need to live inside college and get the best rooms.