Bowser (SSBB)/Up aerial
Bowser performs an upward-swinging headbutt. While very slow, up aerial is very powerful to compensate, KOing fighters at rather low percents. It is the second strongest up aerial in the game behind Zelda's up aerial.
However, outside of its high power, up aerial has multiple downsides which make it a rather unspectacular move. The move lacks range (although Bowser's head is intangible when performing the move), making it very difficult to land against grounded opponents. In addition to this, it has high landing lag and it cannot auto-cancel from a short hop, making it easily punishable. This makes the move relatively useless in neutral unless the opponent is above Bowser. Even in this situation, the move's slow startup (frame 16) combined with Bowser's slow jump squat makes this easily avoidable.
One matchup where the move is useful however is against Jigglypuff as Bowser can force Jigglypuff into an aerial grab release and he can then land a guaranteed up aerial for a very early KO (especially considering Jigglypuff's low weight and falling speed). Due to his short grab release animation, Bowser has just enough time to dash up, jump and up aerial Jigglypuff before it can act, despite how slow Bowser's up aerial is to perform. This is one of the main reasons why Jigglypuff is considered to be one of the only characters Bowser has a winning matchup against.
Overall, Bowser's up aerial is one of the least useful in the game (outside of the Jigglypuff matchup) as while it is powerful, it is simply way too slow and laggy, which makes it very difficult to land and it is very punishable when it does not connect.