Of all the consoles/handhelds you still play, which is the oldest, and what is it about that system that keeps it in use after so long?

My Atari 2600. What keeps me coming back is the quick, almost ios-like nature of the games. I can pull that thing out, try for a high score and put it away in under ten minutes. It's plug-and-play defined.

My PS1 beats out my N64 by a year lol. From that iconic boot up screen to the legendary games whether it was the Crash Bandicoot trilogy, Spyro trilogy, Wipeout trilogy, Twisted Metal quadrilogy, Syphon Filter trilogy, Metal Gear Solid, Dino Crisis, Resident Evil trilogy, or the endless great licensed games from the likes of Disney, Pixar, or Warner Bros (namely Looney Tunes) there’s so much to enjoy! And while gameplay is aged, there’s an undeniable fun factor still!

The PS4.
I have hardly touched anything but PS4, Switch and PS5 for a long time, and even PS4 and Switch have been used just a few times too.

I pretty much only have time to play Genshin Impact and Zenless Zone Zero for months now, I'll try other games here and there but two gacha games are way too much of a time sink.

I may drop Genshin once more as Nod-Krai is coming to purposedly artificially extend it so they can sell more characters for a full year, instead of finally moving to Snezhnaya that has been the point for all these years so we can finally understand the fatui plot, so I may have way more time to get back on older games again. Plus I finally finished university too.

Genesis: Sonic

I keep going back to the Genesis to play Sonic games. It's the oldest system I always go back to for that reason.

In the past year or so, only as far back as Xbox 360. Just great vibes all around, SoulCalibur IV, and some other titles.
I haven't used GameCube in years, but for a while that was the answer. I do play GameCube and Wii at one of my family member's houses sometimes.

My PS3 because despite recent progress emulation compatibility is still lacking.

Through emulation, the TRS-80. I have an actual one of these in storage in some musty box, who knows if it still works. The floppy drive wasn’t working. But it has three games I remember from my youth that are still fun to play: Penetrator (Scramble clone made by an Australian firm), Galaxy Invasion (Galaxian clone that I actually like better than the source material), and Bee Wary (game where you’re a bee trying to sting a spider.)

Also have emulation of two other early 80s computers I enjoyed. Texas Instruments TI-99/4a and Atari 8-bit line. For the latter, I also have the Atari 400 Mini that Atari released last year.

NES. Grew up with it and just love many games for it.

The Xbox 360 is probably the most modern retro console I use on a semi-regular basis, but it's the SNES that eats most of my retro gaming pie.

Outside of consoles... I do have a "retro" PC with 3dfx Voodoo 2's and a Radeon 9550 sitting with an AMD Athlon 3000+ for 5:4 aspect Windows 98 gaming.