In 2006 Nintendo tried to bring back old school Mario platforming and New Super Mario Bros smashed on the scene to great reviews. I'd say it's a nearly perfect Mario game in that it's fun and feels challenging enough. The first handful of Worlds aren't terribly difficult but they are inventive and fun. The last few Worlds, 8 in particular, ramp the difficulty up considerably which makes beating them all the more rewarding.
Nintendo brought Mario to iPhones and Androids recently and I much prefer a traditional play style Mario game like New Super Mario Bros to the tap to play Mario Run. Mario Run is just about the best you can do on a phone but New Super Mario Bros is some of the best Mario platforming around and the traditional style d-pad thumb controls on the 3DS really make it shine.
If you have a DS or 3DS I recommend adding New Super Mario Bros to your collection. The "New" Mario platforming series has received a couple sequels such as New Super Mario Bros Wii, New Super Mario Bros 2(on 3DS) & New Super Mario Bros U. All of those games look great and I plan on picking them up at some point. It seems Nintendo is hip to keep the "New" Mario platforming franchise going and I expect that they will bring a new game to the Switch at some point to keep the old school platforming alive for their new shiny system. And if they do and you have a Switch I recommend you look into it because if it's anything like New Super Mario Bros it will be well worth playing.

NSMB Wii is amazingly awesomely good. The DS game is great, don't get me wrong, but the one on Wii ramps everything up. It has that same satisfying difficulty curve and tight platforming.
Yeah, it's on my list to get New Super Mario Bros. Wii next! I do remember it being really hard though from when I borrowed it from a friend.