A lot depends on why you play retro games. Is it purely nostalgia or is it about having a larger understanding of the medium? Different people are going to have different reasons for playing older games and those reasons will define what they like to play.
I had a discussion elsewhere with someone lamenting that pre-3rd gen gaming was ending up all but forgotten. I tried to explain that you can't expect everyone to be on top of the entire history of gaming. You can't expect everyone to have the same interest in playing and appreciating everything for what it's worth. It would be like expecting all movie fans to have an encyclopedic history of early film. They were conflating popularity with historical significance.
Now for me personally:
As a gamer and someone invested in gaming history I have no range. I find it all pretty fascinating. I love to analyze home ports of Golden Age arcade titles and how they evolved over the course of the 2nd gen, slowly pushing closer to arcade perfect. I like to see interesting game mechanics and concepts that people tried that either didn't work or didn't leave enough of an impression to stick around. I also like seeing where certain game mechanics started, thrived and influenced future generations.
The best games of the 2nd gen are still good, fast, pick up and play games that are easy to play and difficult to master like all good arcade titles. Early attempts at deeper games like adventure titles, RPGs and simulations may not have fared as well but with a little patience some are still quite playable.
As a collector I don't do much earlier than the NES except for the Atari 2600 as it's the 2nd gen console I had access to as a kid like most people. Older consoles are getting more and more inconvenient to collect for as technology marches and with just about any electronic device able to easily emulate them I was never really interested in building up a physical collection. I also like mucking around with Plug 'n Plays for at least a facsimile of the real thing (I got the ColecoVision one recently almost solely for the home port of Frenzy).
I find a lot of first gen stuff really cool, even though most are variations on Pong there are some interesting standouts. Collecting them would be kind of a hassle though and they'd mostly just be conversation pieces. I would like to get a nice wood grain Atari Video Pinball someday though, if I'm ever going to own a piece of the 1st gen.
