That's actually a good question, what is the best of the small retro handhelds? Not concerned about price, just build quality and overall ability to game (battery life, screen size/type, control quality, that sort of thing)
My wife really wants one to play her favorites on and she doesn't really like my Steam Deck (which is what I use) so I'm hopeful I can find something smaller and yet still accessible.
Unpopular opinion among retro fans but I'd shoot for an Android handheld regardless.
Everyone and their grandma is used to Android by now. A simple launcher like Nova, with per-game shortcuts on the home screen, is more approcheable and familiar than even the most well-meaning frontends for Linux. Odds are she already plays Android games, so there's nothing new to learn: tap the icon to play, use the task manager gesture to close.
The way you word it implies your wife is interested spontaneously, which is already a step up from the retards trying to force grandma to play retrogames, but you still shouldn't overestimate how much novelty and effort she will be willing to bother with.
If she wants to play "her favorites", I'll assume that means a relatively small collection, at least at first. 8 games in a 4x2 grid is already plenty, and since Android is simply plug-and-play to a PC for sharing files, these games can easily be rotated with new ones if need be.
Be sure to pick a nice wallpaper too, first impressions go a long way.
Pictured: my wife's Odin Lite. The first row are the games she plays regularly on her own violition. The second row are the rotatable games she plays (or drops) on my suggestions.