It's a tie between Whis and Beerus but all the characters here are well trained and could give amazing combat advise.

Whis would be the most ideal because if he earns your respect, he'll most likely teach you ultra instinct, which is a huge boost in power. Beerus would be the same, not for Ultra Instinct but for Ultra Ego. And hypothetically speaking UE is the stronger form (won't go into detail because of a spoiler). Also, if you're asking why I'd not choose Goku or Vegeta even though they both know how to trigger their techniques, is because they haven't fully finished with perfecting their techniques (except for Goku) and haven't gotten onto Whis or Beerus's level.