I wrote this on some other post: I got my hands on the patch for the SMB3 GBA port where you can now access all 38 levels from the eReader cards. They pulled the levels from the Wii U VC release.
Anyway, those levels are something else! Highly recommend it!
Edit: As far as the best Mario game — it’s quite tough but for 2D I’d give the node to Yoshi’s Island. For 3D, it was Galaxy 1 & 2. But recently I’ve been playing 3D Land and 3D World and realized that I enjoy a more linear Mario game. Those two have lots of levels that are short and sweet.