At first I was going to say SNES, since I did not own one as a kid and I'd have the chance to play all the 16-bit games I missed, but instead I'm going to say PS1. The classic SNES games are being re-released in various services or packs including Switch Online in their original form and if push comes to shove SNES emulation has been quite good for a long time. I've played even less PS1 and it has a huge library full of "hidden gems" and classics in all sorts of genres, many of which may never see a re-release on retro consoles or services. Just pick a type of game you want, the PS1 probably has it, something that even the SNES may not be able to say.
Oh I'm not saying that the PS1 has the "best" library, but among the 16-32 bit console generations it likely has the most varied library of games that are generally more difficult to emulate/play properly on other things like emulators and which is likely to be in more danger of losing more of its library to the aether of lack of preservation than the Genesis, SNES or perhaps even the N64. Also, while you can find the classic PS1 games such as FF7/8/9, Twisted Metal, Castlevania SotN, Metal Gear Solid, Crash, or Spyro all over in collections or remakes like Crash N-Sane Trilogy and Spyro Reignited, there's a lot of weird and fun experimental stuff on the PS1 while devs were figuring out what they could do with 3D and mastering 2D gameplay that I would absolutely love to give a try. There's a certain aesthetic of 2D PS1 and Sega Saturn games that has always looked oddly charming to me and I enjoy.