Generally speaking it's best to fit radiators beneath the windows of a room. The reason for this is twofold:
Firstly, cold air falling by the window will be balanced out to some extent by the warm air rising from the radiator. Fitting them on the opposite wall can actually cause a noticeable draught, since warm air rising on one side of the room and cold air falling at the other exaggerates the flow around the room. (There's an illustration on the website.)
Secondly, and purely a practical consideration, is that for the most part, the wall area beneath a window doesn't tend to have furniture in front of it. You therefore make use of unused wall space and, at the same time, do not end up shielding the radiator with a chair or the like.