Honestly I think they're all equally wonderful, but some may be less accessible than others.
For instance, while I love Mermaid Melody very deeply, I think it's a big undertaking to watch because it's nearly 100 episodes long despite being a fairly vapid show made for particularly young children.
It's also hard to recommend any show that only has very low-quality fansubs and no official subs, for example, Powerpuff Girls Z. Good show, but the only subs out there are TV-Nihon, and the official version is just a rewrite dub-only thing...

I would recommend Cardcaptor Sakura to absolutely anyone, very openly. It IS as good as they say and every aspect of it is largely great. (I just don't like the main canon ship...)

There are a lot of shows that try to deconstruct Magical Girl shows as a genre and do so poorly, or that satirize specific things about them in ways that show they've never actually watched or read a Magical Girl work. I would avoid stuff like that, and there's a surprising amount of it out there.

The only deconstruction I'd recommend is Madoka, and it's definitely in the "watch this after you've seen some other Magical Girl shows first to understand the genre and get the most out of it" category of things.

I can't recommend the sidestory to Madoka, Magia Record. They did not know what they were doing when they tried to make an anime out of that game. It's a good story at base, but the anime is a confused mess. Only watch it if you really want more Madoka.

Magic Knight Rayearth is a classic that sits on a line between isekai, magical girl, and mecha. It's very strongly worth a watch, as long as you watch the series. The OVA is a very strange inversion that doesn't work well. It has redeeming elements but should be avoided. But the series? A must-watch.

One of the strangest adjacent things I ever watched was a show called Ultimate Girls. It's like Ultraman if it were an ecchi show where girls transform into giant girls and their clothes melt off. Cannot recommend, although the music, specifically, is good. Amuro Ray's original voice actor plays the perverted mentor character. As a result of the transformation sequences and such it is Magical Girl adjacent, but what even is this show? How did it get greenlit? Avoid at all costs.

Avoid Naria Girls. It feels more like a confused parody skit than anything else and it makes me cringe if I am being completely honest with you -- something that VERY few shows do. Like, period.

If you want the classic recommendation, all versions of Sailor Moon are good. Yes, even the live action one, it's just very different. They are all different beasts. Try them all. See which one you like most. Sailor Moon is what got me into the genre, as is true for many people. Remains an easy recommendation.

Seriously, no matter how many things I watch of the type, I feel like Cardcaptor Sakura is always going to be the absolute top recommendation. It's the Magical Girl show. Best in class. Even with so many things being made over all these years, I'm not sure anything will ever truly outdo it.

But my understanding -- advice I have long avoided despite my love for Magical Girls -- is that you should also check out the PreCure shows because there's something there for everyone. Maybe I'll watch them someday instead of only watching literally everything else.