I think it peaked at Super Mario World, with the snes.
The following "new" titles of super mario bro's hasn't been up to the same level (or maybe im just old and jaded).


New Super Mario Bros. 2 + New Super Luigi U feel especially bad imo.
Its like nintendo doesnt care to put in the effort in these games anymore.

Super Mario World is easily my favorite. The aesthetics were pleasing, the music was great. The gameplay and level design were fun. Yoshi and spin moves were decent additions. Beat the game more times than I can remember, and probably my #1 played SNES game.

Honestly just focus on doing good quality 3D mario games instead.
If they do have to do a 2D one, it needs to NOT feel like a "lazy effort" level of game.

I also dont wanna see mix/match type (hybrid 3d?) thing, like super mario 3d world.

Do one or the other, dont go half ways and stop, trying to please everyone, while only giveing them half of what they wanted.
Either full 3D type mario, with free movement ect, or a 2D platformer.

I honestly find Super Mario World to be one of the lazier efforts when it comes to level design. It is just lacking in creativety. The soundtrack is also extremely repetitive.
I like some of the New Super Mario Bros. games better. The peak of the series is definitely Super Mario Bros. 3 though, it is so packed with ideas and charm, it is pretty much perfection. It also has the right level of challenge for my taste.

Super Mario World with SMB3 as a very close second.

New super mario bros U. It gets a lot of hate but I think it was the victim of over saturation. The core game is really quite good.

Super Mario Bros. 3 is still the best game in the series in my opinion. Mario World has the better graphics (obviously), but SMB3 has more varied levels, stellar gameplay, feels like an actual grand-scope adventure and the power-ups are amazing.

Bonus question: I definitely want the next SMB game to have better production values. The "New" series (after the first game) basically felt like B-titles Nintendo didn't really want to develop. The next game should feature more world-building, more playable characters (what about Daisy?), be of grander scope (exploring actual kingdoms instead of just generic "worlds") and modernize the gameplay. Maybe a more open-world approach would work if done correctly? Maybe adding more "adventure" elements would make the games feel fresh. I want the series to get rid of the never-ending monotony of "grass world, desert world, ice world, jungle world" theming. There is so much they could improve! I don't want a story-focused title but the player's actions should tell a story when playing a Mario game. Right now they don't because the games feel like they were made in a sterile level-editor.

There's so much potential in this series!

I'm gonna go with Super Mario World as well. That's the game I played the most. If I had to choose one for handhelds too, I'd choose Super Mario Land 2.

Super Mario Bros 3 is objectively the best game.
Though I'm more attached to Super Mario World (the 2nd best) due to nostalgia.
Super Mario Bros 2(USA) is different but definetly deserves more love.

Does Super Mario Land count?