tl;dr: Miyoo Mini Plus

Performance-wise, pretty much anything is gonna have you covered when it comes to GBA, so it really depends what else you want out of the experience.

You mentioned the slot, so if physical carts are important to you, then...

• If you want an authentic feel with modern features, then an OG GBA mod is probably the "platonic ideal" if you have the stomach for it. WULFF has a good video about his experience: https://www.youtube.com/watch?v=OhW6TkzmxuM
• Analogue Pocket is also an option, but honestly I think they're ugly and overpriced.

Worth noting for the platform-specific perks:

• A hacked 3DS is a great option. It's super easy to do, and it's really the only hardware that comfortably plays 3DS titles to this day.
• Hacked OLED PS Vita is also a great option. Beautiful screen, great controls, and again it's the best hardware for its own first-party titles.

More modern stuff:

• If you also want Moonlight streaming and/or Android apps, Retroids are great. I have a 3+, but I wish I'd waited for the 2S (the numbering in this hobby is ridiculous) because I would've preferred the 4:3 screen ratio and retro aesthetic.
• Anbernic is hit or miss -- but when they hit, they hit. The RG35XX H is reportedly a very good horizontal option, and I've had my eye on a 405 M as well because of the thumbstick positioning.

But here's the thing: At the end of the day, the best handheld is the one you have on you.

If you're not going to bring it with you when you go out, then what's the point?

Portability is the key feature of this kind of device, and we all tend to have at least 2 if not 3 pockets already spoken-for. And you might think "Ah, I always have my backpack on me." But do you bring your backpack to the bathroom with you at work? Do you wear it around the house? So pocketability is the #1 factor, IMO.

And with that in mind, here are my pocket-friendly picks:

• Anbernic RG280V is the epitome of the "just enough" device. Just enough screen, finger space, and performance to play just enough systems to keep you happy.
• Anbernic RG Nano is an extreme take on pocketability, being small enough to put a keychain. I include it here just because that may be a relevant category for some people.

But my number one pick, specifically because it nails the form factor while being supremely pocketable...

The Miyoo Mini Plus!

It's just a stellar device. It's just big enough to be comfy, but small enough that I don't hesitate to put it in my pocket even for trivial outings.

Onion OS is a great custom firmware (oh yeah, for a lot of these you should look into custom firmware options), but the stock firmware isn't bad either. It's easy to use, everything "just works", and it's plenty powerful for everything that I care about playing on the go. The screen is beautiful, the gray version looks cool as hell (be aware that there is light bleed around the power LED with the translucent cases), and the stock buttons are delightful.

And it's a good price, to boot.

I bought one for my sister-in-law because she was drooling over mine. She's not a super nerd, has zero interest in tinkering, but hasn't gone a day without playing it since Christmas.

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Some good channels for in-depth reviews:

• https://www.youtube.com/@RetroGameCorps
• https://www.youtube.com/@TechDweeb
• https://www.youtube.com/@WulffDen
• https://www.youtube.com/@ETAPRIME