There are alot of varibles that play into that. For example do you want a cd player to play stock speakers or what. That will change the amount of watts that you want. If you are trying to run a component system or other bigger types of speakers then you might want to get one with no out put and run a nice amp to them. Other factors are what type of vehicle do you have and where do you live.
Exe: convertible and High theft maybe a cheep one, one with a detachable face, or a hard to see face like the JVC Elcamelion. Another factor is you dont want a cheepo in a truck or a Jeep or any thing that you plan to run a lot of bass in, the skipping will drive you crazy. But for a general deck to be used in a car with stock speakers I would get a Sony, JVC, Clarion, ect... about 45 or 50 watts is ideal and detachable face. I good way to judge is anything over $180. You pay for what you get.
Good luck,
2 JL W6's and a MATTS amp.