RetroGaming

I'm surprised no one mentioned the Analogue Pocket, especially if you had cartridges. It does have support for ROMs as well. One of the best features is that you can sleep your device on even original cartridges

I find playing Nintendo games is always best on Nintendo systems. I use a hacked 3ds for all my handheld games including Gameboy, a hacked Wii U for Gamecube, Wii, and Wii U games, and a hacked Switch for everything else.

The PowKiddy Pocket 90 is fantastic, plays 16 bit and older games very smoothly, and has the nice clamshell design of the Gameboy SP, making it very practical for actually carrying in a pocket.

The one drawback is it gets unstable when the battery is low. But being aware of this issue helps.

IMHO with an average Android phone and a 20/30$ gamepad, is your best bet. I use an Ipega Red Spider and I love it!

All Gameboy Advances play GB, GBC, and GBA. The best GBA is the SP model AG-101 which has a factory backlight screen. However, there are tons of modifications available for Gameboys out there. I say get a used GBA and go to https://www.retromodding.com/ and go to town on it. Customize it the way you want

For ROMs only, the absolute best way to play is to get a Nintendo 3DS or 2DS and install home brew software. And as a bonus you can play the entire DS and 3DS library as well.

FYI the Nintendo DS can play GBA games, but it does not support GB or GBC.

I know you said you wanted handheld, but for anyone else reading my absolute favorite way to play is with a Gamecube and a Gameboy Player! It actually plays all GB, GBC, and GBA games! Gameboy Players can be cheap, but the boot disc is expensive. To get around this there is home brew software for the Gameboy Player that is very good, although I haven't tried it myself. To use it you need an Action Replay and a cheap SD card to GC memory card adapter. The software will boot the Action Replay first, then boot whatever is on the SD card. (I bet there are ways to load roms too but I haven't looked). I play with a Raphnet SNES to Gamecube adapter.

Miyoo Mini plus and Ambernic RG35xx are probably what you want. I have these two devices and the 353V, all three are quite nice but the MM+ and RG35xx are better to your needs and are probably better for beginners.

Personally I would go for the MM+. Definitely install OnionOS if you get a MM+ or GarlicOS if you go for thr 35xx. (JELOS is my pick for the 353V) These custom firmwares are far more polished and have more performance than the default OS and have tons of great features and community applications.

I play GB, GBC and GBA games on my moded 3DS with TwilightMenu++.

Powkiddy V90. It's got the GBA form factor, a good and same proportion screen. reasonable software and CFW support. Reasonably priced and a very easy to replace battery if it should be needed.

Been really loving my Miyoo Mini Plus since OnionOS has retro achievements.

I'm currently playing the GBC game Oracle of Seasons on a PlayStation Vita. It can run everything up through N64 rather well.

A huge recommendation for the RG35XX. I got a the grey one and a DMG style skin from Sakura Retro Modding on Etsy and it's perfect for what I wanted. Finding some decent overlays for the dot matrix grid is easy, and it all plays perfectly.

I would either use a hacked Switch, or a Steam Deck. What about the Deck isn’t selling you? It’s a wonderful emulation machine, although definitely a bit pricey if that’s going to be the majority of your usage

If you have an Android device: Get the Gamesir X2 Pro. It turns your phone into a Switch-like device. It even has a USB-C connection, so there's no input lag. I love this thing!

GB emulation is one of the lightest, so you are drowning in choices. YouTube channel Retro Game Corps has a number of great videos and comparisons for both high and low end emu devices. Check one of their buying guides, it may give you good ideas! https://youtube.com/@RetroGameCorps

I have the miyoo mini and love it for casual always in my bag gaming. The plus might be better for longer playtime due to comfort. Miyoo is great because it has a pretty large community with customising to onion os for qol features.

Or just grab a flashcard for a Gameboy Advance/Color. Preferably with a backlight modded screen.

I love my little rg35xx !

I bought a gameboy color and a EZFlash cart and can play any GB or GBC games on it.

My one was pre-modded with an IPS screen but I have also modded one myself and it wasn’t overly difficult.

There’s something about playing on an original Nintendo machine lol

the question I would ask along these lines is which systems have a screen that can do a perfect integer upscale for GB games? so that the pixels are each doubled or quadrupled etc without ugly scaling issues.

Every handheld made in the past few years plays GB/GBC/GBA at 100%.

I have both am R4 card for my 3DS and a 353V. Loving the 353 as it covers so many systems. Ripping all the games from my Dreamcast from back in college has been awesome. GB, GBA are flawless on that device