Difficult, shmifficult. You guys don't know what difficult really is. I may not be able to beat C-4 Bowser with fire except for once, but it could be harder.
Anyway, I have to vote for Super Mario Bros. 3. It pretty much epitomizes the whole series, and everything great about it. A lot more level variety than most of the other games in the series, except possibly oddball Super Mario Land 2. But it had the airships! No other Mario game touched those, and I couldn't tell ya' why. And a lot of great enemies introduced here, or just for the one game, like Chain Chomp, Nipper Plants, Boo, Thwomp, the Hammer Brother family--the enemy quality took a big nosedive in Super Mario World.
And all the power-up suits in Super Mario Bros. 3, with the leaf, tanooki, hammer, and the... actually, they probably should've tweaked the frog so it didn't absolutely horrendously suck on land. Ah well, so it isn't perfect. Still as close as we're gonna' get!