I would say Mario 3.
the ability to stockpile items and all the cool items you could collect
i.e Mushroom, Flower, leaf, power wings, Tanookie Suit, Hammer Brother suit, frog suit, Flute, Lakitu.

I did find Mario 3 more difficult than Super Mario World also.

Remember the world 8 Ghost House. I often used to use Lakitu on it as I often got lost in it so would skip it.

Is it me or were the kooper kids more of a challenge in Mario 3 than in Mario World?

Super Mario World just holds better over time because of the way it looks it looks better than a lot of games that came after it.

Also I prefer Super Mario World, it's funner and more memorable to me too.

Well, if we're going to put looks into the equation, let's look at Super Mario Allstar's SMB3.  Although SMB3 looked GLORIOUS on the NES.  Geeze, I was just playing it these weekend and I was like "Wow, this is on the NES?"

 Super Mario World did have great graphics though.

Nintendo needs to put Mario All-Stars on the Virtual Console. It's a much, much better way to experience Mario platforming in all its glory.

Super Mario World, IMO still the best 2D platformer for home consoles.

Super Mario World for me but why didn't they keep the Raccoon Suit turing to stone was cool.

Super Mario World my favorite but Super Mario Bros 3 a close second

SMB 3 = one of the greatest platformers of all time.