I say 2600. It has the most shovel ware, but also the largest library, and games can be found pretty much anywhere. (Thrift shops, retro game stores) The 2600 was also badass strong, you could throw it against a wall and it would still work. I heard InTV's just stop working for no reason. What a pile of piss.
Or just go for the 7800, not only do you get the entire 2600 library but you also get 7800 games, that are better versions of 2600 arcade ports. AND you also can own and play the immortal Ninja Golf.
Ninja Golf was the greatest video game concept in the history of all video games. It puts Ys I&II to shame.
lol