In most cases it makes sense to pick the supervisor and then go wherever he is, rather than picking the university. I've heard people say that the relationship a student has with his supervisor is the second most important one in his life, after the relationship he has with his spouse; I've also heard people dispute this and argue that the supervisor is more important.
Of course, in order to be in a position to pick a good supervisor, you need to make yourself attractive. Do research; publish papers; go to conferences and present your work there; talk to other people in your field. Faculty are always looking for more good graduate students, and meeting them at conferences will give you a chance to figure out if you get on well with them at a personal level.
That said, I did almost none of the above -- but I was a special case, in that I didn't particularly need any supervision (I'm pretty certain that I literally saw my supervisor less than two dozen times while I was a graduate student, and interacted far fewer times than that).
