I have too many handhelds for my own good. I never sell anything, so I still have my DMG Game Boy, PSP, DS, Vita, etc. Anbernic releasing a new handheld practically every month didn't help either. But for now, the last handheld I bought from them is the one I pick up the most: the RG35XX SP.
It can run the same games as my other emulation handhelds, but the form factor combined with the OS makes me use more often. It set it up with Mustard OS (muOS). Whenever I want to stop playing, I just fold up the handheld and it'll go to sleep. Leave it alone long enough and it'll shut down, but not before saving the game's state. When I boot it up again, it'll resume my game. As a bonus, it can even function as a video player (via MP4 files on the SD card) and looks like a tiny version of those portable DVD players back then. It really feels like something from Y2K.
Its only flaw is that the R1/L1 buttons on mine are not very responsive, and I hope they don't fail eventually like the R1 button on my RG351P (I plan to try resoldering that sometime).
Another one I use often is the RG ARC-D. Nearly all of these retrogaming handhelds mimic Nintendo-style consoles, but this is one of the few that actually offers a Sega-style button layout and form factor. It's perfect for Mega Drive/Genesis, Game Gear, and Master System. The dpad also makes it ideal for classic shoot-em-ups and other ARCade games, which is what Anbernic was going for when they named this.
Finally, I bought a Steam Deck just last year. As a handheld enthusiast and Linux user, I'm still tinkering with it and seeing what I can make it do.