Of the three, CV, 5200, 7800, I prefer 5200 due to its coverage of arcade games that I really liked. I have to say though, that having 5200 and CV side by side about covers the 80's arcade scene as good as it can be.
The 7800 is a decent system, but to me seems to have sub-par controllers to 5200 and CV. I know you probably have heard all the bad things about 5200 controllers, but a good refurbed set on a properly calibrated console make for a nice gaming experience. Plus, there are things like PC 15-pin controller adapters and master play devices that really open up the controller options on 5200.
With that in mind, you really cannot go wrong with either 5200 or CV. 7800 will leave you wanting for more though as the library is quite limited.