It is difficult to say which character is the "easiest" to learn in Super Smash Bros. Ultimate, as different characters may be more or less suited to different playstyles and skill levels. That being said, here are some characters that may be considered relatively easy to learn for new players:
Mario: Mario is a well-rounded character with a fairly simple move set and good mobility. He is a good choice for players who are just starting out and looking for a character that is easy to pick up and play.
Donkey Kong: Donkey Kong is a strong and powerful character with a relatively simple move set. He is a good choice for players who prefer a more straightforward, aggressive playstyle.
Pikachu: Pikachu is a fast and agile character with a fairly simple move set. He is a good choice for players who prefer a more technical and precise playstyle.
Jigglypuff: Jigglypuff is a lightweight character with strong aerial attacks and good recovery options. She is a good choice for players who prefer a more agile and aerial-based playstyle.
Overall, the best character for you to learn will depend on your personal playstyle and preferences. It's important to experiment with different characters and find one that you are comfortable with and enjoy playing.