Atari 2600 is the most well-known model, it is the most common one, and it has the most games made for it. It is also the cheapest one, and the most primitive one when it comes to graphics and sound. The Atari 7800 is equivalent to an NES graphically (well, maybe just below NES), and it also plays the Atari 2600 in addition to the 7800 games.
I think that the PS4 is probably the best console Sony has ever made, but the exclusive game support is pretty sad. Thank goodness there are some fun multi-plats to play on the PS4. If Sony would ever start releasing some true interesting exclusives, the PS4 could be their greatest console.
Buying a TurboGrafx-16. You can pick up a PC Engine of TurboGrafx-16 on eBay for around £70-150 depending on condition. If you're buying a Japanese or US machine you'll need to also purchase a stepdown transformer so you don't blow your console up when you plug it in – these are available online for around £30.
1 Nintendo 64. The system brought many of our beloved franchises (such as Mario, Star Fox, and Zelda) into the 3rd dimension so well, that it's hard to not say that it's greatest system Nintendo has made. It's innovation is in it's technology. All the games I mentioned above helped revolutionize gaming in some way.