About the Nes, Snes, Mega Drive and older consoles.
I am in the camp who doesnt have much interest in the 2600 or pre-Nes stuf - but I absolutely love the Nes, and i think its probably my favorite system of all time. Like others have said, there are so many great action-plattformers on the system. And while many of them are great in their own right, i also really, really like that the Nes came out before video game genres were clearly established - meaning developers experienced with a lot of games that later would be considered genre hybrids. Think of stuff like Guardian Legend, Blaster Master, Zelda 2 etc. So on the Nes we have the top games that everybody knows and loves (Contra, SMB3, Mega Man etc), then we have the wierd genre hybrids, then we have the more experimental stuff like Metal Storm and Bionic Commando - and the we have just a ton of really great standard action-plattformers like Power Blade, Shatterhand, Shadow of the Ninja, Astyanax and so on. Then we have the rpgs, shooters and other genres. And if we throw in the Famicom - i think you can easily get to 150 and maybe 200 worthwhile games on the system. So all in all, i just think the quality of the best games are great, the quantity is immense, and the variety is also really unique on it.
Now for the Snes and Mega Drive, they are probably my second and third favorite consoles of all time and i love both of them. I think the best games on the Snes are probably the best ever - Mario World the best Mario, Zelda 3 the best Zelda, Super Metroid best Metroid, FF3 best FF etc - and the console also has good variety. But i personally feel that the middle-games - stuff like the disney-games from Capcom, most of the beatemups on the system, the shooters - are generally less interesting than the similar middle-games on the Nes. Meaning that really diving into the library is less fun. For the Mega Drive on the other hand, i feel the top games are not as good as on the Snes, but that the middle-games are often way more unique and interesting, so it is more interesting to dive into for me now.
All in all, i think all three machines are awesome, and for me at least, gaming was at its peak at that time around 1990 - which is probably fitting considering I am born in 84.