Mario (SSBB)/Down tilt
Mario performs a leg sweep. The move has decently fast startup lag, coming out on frame 5 and the move has surprisingly decent range, with the move being slightly disjointed. The move deals 5% on Mario's foot and 7% on his leg/body, with the move sending opponents upwards with very weak knockback.
However, the move's main issue is the combination of its high ending lag and low knockback. The move has an FAF of 35, making it easy to whiff punish and punish on shield, considering its short hitbox duration. The move is -28 or -27 on shield depending on which hitbox connects. This is made worse when combined with the move's weak vertical knockback. At low percents, the move can easily be punished on hit, with some characters even being able to land some of their stronger attacks or lead into their chain grabs. At mid percents, the move can still be punished on hit by numerous members of the cast, with the move not becoming completely safe right up until higher percents. Even when Mario can use the move safely, the opponent is sent too far away and Mario has too much ending lag to do anything. The sweetspots are also positioned closer to Mario and have lower priority than the sourspot, so the weaker and more punishable sourspot will usually connect if the move is used.
Neither of these factors alone are inherently bad. Meta Knight possesses a very weak down tilt in terms of power but it is much faster, has a horizontal launch angle to allow for locking and tripping and it has much more range which justifies the low knockback. Snake's down tilt is a similar speed to Mario's and it launches opponents vertically but it is much stronger (being safe on hit at lower percents and a viable KO move at higher percents) to justify the high ending lag. The issue with Mario's down tilt is that it has both high ending lag and low vertical knockback which do not mix well together at all.
Overall, Mario's down tilt is easily Mario's worst move, as using the move actively puts Mario at a disadvantage. If the move whiffs or is shielded, Mario is left in a disadvantageous spot but even if he connects the move, his opponent still has an advantage over him. Mario can flat out risk losing a stock or at least take a lot of damage if he lands the move. This makes Mario's down tilt completely useless, as his jab and forward tilt are faster and safer spacing moves, while up tilt is a safer and more effective move at close range. It is not only considered Mario's worst move but it is even considered to be a contender for the worst move in the entire game, as there is never a reason to use the move and when it is used, it puts the player at a disadvantage, even if the move connects.