Smash Attacks are your main killing tools. They do high damage and knockback, but in return are usually slow and stiff, sometimes with low range. The best ones, are the ones that don't follow these normal "rules" that are usually there. However, sometimes the best are ones that excel in the strength of Smash Attacks, but still follow those restrictions.
This one goes against the general rules of Smash Attacks, by the entire thing lasting only two thirds of a second. It's fast, strong, and quick to get out of. The only main problem, is the range of it. However, this isn't much of an issue. It's so fast, it's one of the few smash attacks you'll throw out in neutral, so you'll hit one eventually.
This move is all of the power of a smash attack, but with more on top of it. The purple sections of Little Mac's hurtbox are super armour hurtboxes. He can't be hit out of the move during those sections. The power is still great, and the speed is really powerful.
So here's even more power, but now with even less speed. However, the green sections are invincible, and the blue sections are intangible. The entire move is massive and extremely strong, so you get a ton out of this.
Olimar's up smash (and most of his moves) change depending on which pikmin is used. Yellow Pikmin is the best for Up Smash, with the massive range and damage potential. It's large and really powerful.
This move is massive and actually really fast. It's great, and even falls down off of ledges. The move is strong and really just good for almost any scenario.
This move itself is fine, but it gets even better when you realize the yo-yo hangs off of the ledge, allowing for some really easy hits. The move overall is still great, but this makes it tons better.