I don't like Super Mario Bros. 3 that much and prefer Super Mario World. I know the "best 2D Mario" title is usually between those two, so maybe it's not that unpopular.
Now, is Mario 3 better than 1, Lost Levels, and Mario 2 USA in my opinion? Definitely. You can tell they polished the formula a good deal. I still think Mario 3 is a landmark title, but I don't find it quite as good as World.
While 3 definitely has more power-up variety than World, I feel like it's a little inconsistent in difficulty compared to World, especially when you hit the brick wall of World 7. Now, while World definitely was challenging around Chocolate Island (also the second to last world of that game) it was still not TOO bad in my opinion. Oh yeah, and no saving in the original version of 3 also sucks (even if this is remedied by restore points / save states, I still don't like the omission of saving in the actual game). It also seems like 3 got impossible levels of hype thrown at it, what with The Wizard basically being made to promote it (and many other Nintendo games).
But hey, maybe I just suck at the game. Mario 3 is by no means a bad game, I just don't really enjoy it all that much.