It is difficult to definitively say which character is the "best" in Super Smash Bros. Ultimate, as the effectiveness of each character can vary depending on the player's skill level and playstyle. Different characters excel in different areas and have their own unique strengths and weaknesses.
In general, the most successful characters in Super Smash Bros. Ultimate tend to have a well-rounded set of moves and abilities, strong recovery options, and a good balance of speed and power. However, even the strongest characters can be countered or outplayed by skilled opponents, and the effectiveness of any given character can change over time as players learn new strategies and techniques.
Ultimately, the best character in Super Smash Bros. Ultimate is the one that best fits your playstyle and allows you to play to your strengths. It's important to experiment with different characters and find one that you are comfortable with and enjoy playing.