As someone who hasn't had that much exposure to the magical girl genre yet, I find that I've tended to think of the "big 3" magical girl shows as Sailor Moon, Cardcaptor Sakura, and Precure, and for a "big 5" add Ojamajo Doremi and Tokyo Mew Mew to the aforementioned... But really, I feel like the main determining factor for inclusion in this list is more than anything just that I've repeatedly heard about them, and that isn't objectively the same as actual popularity and influence, although there is certainly a correlation.
So what would your lists look like by comparison?
As someone not really qualified to speak on this, my impression is that the two magical girl shows most casual anime fans would be familiar with are Sailor Moon and Madoka Magica
Edit: Sailor Moon and Cardcaptor Sakura both benefitted from pretty extensive localizations, Sailor Moon being comparable to Dragon Ball Z in terms of success. Madoka came out during the height of the streaming boom, and was basically Evangelion but for girls.