I couldn't really find a guide online that would have pointed me in the direction of the "best" ending without mentioning any major plot points, so here's my two cents. It's also worth noting that the "best" or "true" ending is entirely subjective. The visual novel is pretty short, so unless you're like me and terrified to make decisions by yourself, play the game naturally and reload to see the other endings.

Song of Saya has two points where you need to make a decision.

At the first decision point, you're given two choices:

• The top one: ...want it all back.
  
• The bottom one: ...don't need it anymore.

The top one will end the game early and present you with what is probably considered the "bad" ending. If you picked the top choice first, go back and pick the bottom choice to see more story.

At the second decision point, you're, again, given two choices:

• The top one: He calls Fuminori
  
• The bottom one: He calls Ryoko

Both of these choices lead to different endings. What I would consider a "true" ending and a more "neutral" ending.
The top choice takes you to the ...true ending.
The bottom choice takes you to the neutral ending.