Super Mario Bros.- It's primitive compared to what came after, but it's still a good time occasionally. The level design is still really good and it just feels very easy to pick up and replayable. It's also one of the most revolutionary games of all time, so that's a plus.
Super Mario Bros.: The Lost Levels/Super Mario Bros. 2- Basically Super Mario Bros. Hard Mode. It has some new features in it, but that's just what it feels like to me.
Super Mario Bros. 2/Super Mario Bros. USA- This is in my mind the better Super Mario Bros. 2 despite being a reskin. It has the most diverse bosses out of the original four platformers, it has four different characters, it added series staples, and it's a unique game compared to the rest of the series even today. If you want to play it then the GBA remake is your best option.
Super Mario Bros. 3- The best 2D Mario platformer. It introduced world themes, world maps, the Koopa Troop's first official bosses aside from Bowser and his imposters in the form of the Koopalings and Boom Boom, new power-ups such as the Super Leaf and the Hammer Suit, non-level areas such as Toad Houses and 1-up minigames, Fortresses, airships, new enemies such as Spikes and Chain Chomps, and much more. Even today the level design holds up. Almost every level feels unique in some way. The game also controls great and provides a great progression in challenge. I love this game even today.
Super Mario World- While I enjoy 3 more, this is still one of the best 2D Mario platformers as well. While the level themes are less diverse and the game doesn't feel as big of a step forward as its predecessor, this is the game that gave us secret levels, Yoshi, extra worlds, more iconic enemies such as Wigglers and Magikoopas, the Spin Jump, and once again even more than that. The game still looks great, and the music (while indeed a bit similar at times) is still very well made.
Super Mario Land- This is admittedly the only Mario platformer that I have not played yet, but from what I've seen the game seems like a solid start for platformers on the Game Boy and later handhelds that continues to add new ideas such as vehicle levels and some of the most unique enemy designs in the series. Regardless, it isn't the best game to bear the plumber's name by a landslide.
Super Mario Land 2: 6 Golden Coins- If Super Mario Land was the boat ride into Disney World then this is the park itself. It takes a lot from world, including a world map, secret levels, the "three power-up" system (Super, Fire, Flying/Gliding), and a similar take on bosses. Regardless there are still new ideas being churned out of the Big N Machine, such as more wild enemies, unique worlds such as one in and around a typical house (while small of course) and one inside a mechanical replica of Mario, and of course now main character Wario. The final boss with him by the way is really good and unique. Overall this is a really good handheld Mario platformer and a big improvement over Land, but it isn't perfect. The game feels a bit awkward at times and it just feels less grand than some other installments in the series.
I'll do the rest later.