I voted for SMB3, even though this doesn't make my top 5 favorite Mario games overall, it is the best out of Bros. 1, 2, and 3 for the NES in my opinion. Don't get me wrong all three are amazing titles and should be in any gamers collection but the first 2 are very overrated. I really like world 6 and the reason being is because of that difficult course in the ice cave, I don't remember the exact level (6-6 maybe?), I do remember it took me a few good weeks to beat, and from time to time I will go back and play the game just for that course alone as it stands out for being one of the most creative levels, I really did enjoy that game and introduced us to the koopa kids as well as many new enemies (that giant fish, what a pain in the arse he was)
Thank you