Very hard choice. SMB3 was the big game when I was a child, I nagged and nagged on my parents to buy a NES, and we finally got it for Christmas, with both Super Mario Bros and SMB3. I played it to death, I still remember all the secrets, and I actually discovered a glitch (or maybe it was an intentional secret?) on my own in the final level. There is a place in the level where you can stand on a statue, but the space to the ceiling only fits small mario or big mario crouching. If you are big mario there and crouch and jump in there, and then stand up, you slide through the wall and get out on the other side, and when you get to Bowser then, when he roars to make fireballs, none come out. It was epic to discover.

But, SMW is probably more replayable, since you can save whenever, and replay your favorite levels as many times as you want. If SMB3 had the levels replayable, it would stand the test of time better, but I think it still has the best overall level design, and the right difficulty level - tough in some places, but not so tough you won't make it if you keep trying a little. Super Mario World is much easier.

And getting the tanooki suit and hammer suit for the first time - AWESOME! Tho since there's only like 1 or 2 places you can get the hammer suit, it was kind of a drag to lose it after wearing it for only 2 minutes :(

Plus, Kuribo's shoe!!!

One thing that always bugged me tho, on the back cover of the Super Mario Bros 3 box, the in-game pictures are from what appears to be the japanese version, and they look different! The overworld design of world 1 is slightly different, AND there is a level displayed there with green grass and those flying creatures that you could stand on, a level that is not in the US or PAL version! Did anyone here play the jap version, are there a lot of differences?

To be honest... i could never choose one :s
Both are simply amazing