A lot depends on how much you are willing to spend, and what sort of running you are wanting to do on it. In my experience cheap treadmills are not worth the money (sub £200), unless you are just looking to use them for walking on.
If you have £500-1000 to spend on one then you are starting to get into the more useful bracket. Personally I've got a Horizon Elite 507 (had it for 6 years) and would recommend it, it has a good powerful motor, goes to 12mph and incline to 12%. Would also recommend fitness-superstore.co.uk, that's where we got our treadmill from and they were helpful and efficient.