That's real tough but I think there is a definitely answer at this point.
Mario is definitely more consistently great. Of the 2D games in the 2D era and 3D games in the 3D era, and of games that were released globally (so not the real SMB2) and weren't just Mario-skinned entirely different games (so not the western SMB2), the only game that wasn't peak gaming was Sunshine. And even it was very good, but had some poor game design decisions that make it worst than it actually plays and can be frustrating to play through at parts.
Zelda meanwhile had the experimental and generally not well regarded Zelda 2, WW fantastic but many people complain too much tedious water sailing, TP cool but by far the worst main mechanic in any Zelda game ever and really represented when Zelda started to lose its luster as a peak action adventure franchise and just started to feel like 'another good game' in the genre, and then that continued with SS which is generally regarded as overall the worst 3D Zelda. So basically a fairly weak 3 entries in a row (at least compared to other Zelda games and Mario games) before BotW resurrected Zelda and made it far more popular than ever and got it back to the top of the action-adventure genre and also put it right at the top of the open world genre.
2D in both franchises is kinda weak and side project-y at this point. Much more interesting indie games in those 2D genres these days. But were the top of the top in the 2D era.
I think Zelda can reach higher heights than Mario, just by the nature of its genre being larger more epic games. But it still hasn't hit those heights, as the tiny 64-bit OoT is still considered by far the best non-open-world 3D Zelda game, and while most rightly consider BotW one of the greatest games ever I think everyone who has played it can point out a few major things that could easily greatly improve the open-world Zelda formula. So Zelda has more room to climb and it has had several less than outstanding entries and the franchise had gotten weak enough where it had to be reimagined and resurrected by BotW.
essentially:
Mario with 2D (SMB, SMB3, SMW) and 3D (64, Gal, Gal2, 3D W, Odys) has many times hit about as high as it is possible to hit in the genre for the given time periods - all 8 of those games are masterpieces as well as classics because they all still hold up phenomenally today. Whereas for Zelda well there would be a lot more debate cuz depending on who you ask you'll hear people say WW/TP/SS/BotW/TotK are anywhere from the the greatest game ever to it's the easily the worst Zelda haha, but I think it's reasonable in general to say only LoZ, ALttP, OoT, and BotW hit at or very close to peak gaming experiences for their time. I'd say those top few Zelda games that are absolute masterpieces tend to be considered better than the many Mario masterpieces, but two-thirds of them belong to Mario, while Zelda has had multiple "it's good, but..." games.
So Mario wins.
