Which system is your favorite??... 2600,5200,or 7800. for me its quite simple...7800 is my favorite for the reason that the games for it rock and it can play the older 2600 games...and the controllers are the same. you can even use sega genesis controllers with it...(at least the ones that I have). The 5200 was an awesome system...but kinda hard to find...especially when it comes to good controllers...I think they should make new ones and sell them if you ask me.
The 2600 of course is probally the most fun....although the games are not as good as the 7800, there are a ton more of them. and they are pretty darn fun.