Good advice from all but I guess it comes down to what you want it for and how deep your pockets are.
I have recently been collecting a few Panasonic CD boxes and I personally can not praise them enough.
They are generally quite cheap and the models I have are all loud and really fill the room.
I have an RX-DS45 and a RX-DT505 which were both under £50, are small and light enough to carry around and sound good.
Next level up are the cobra tops, I have the RX-DT75, its a bit bigger and heavier but it sounds fantastic. But it will cost you more money.
After the cobra you are in 3 piece territory, DT610's can found fairly cheap and are loud with quality sound and are small enough to carry about.
Then the top of the range DT680, I have not heard one of these but from what I have read it is almost HI FI quality sound. These are starting to become quite popular though so if you see one in good condition and good working order for anything under a £100 I would grab it