What is the easiest but best way to fill in holes before repainting?

Use spackle, then sand flat.

Tiny little tubs of spackling are available at hardware stores as well as Home Depot, Lowes and paint stores. I use a small flat blade knife to apply, let dry, then lightly (very lightly) sand. Tada! Ready to paint.

Vinyl spackling and your finger for small holes in drywall. Wood filler and a putty knife for holes in woodwork.

White toothpaste works well.

Spackle. But here's how we did it when we were commercial painter's. Get a little amount of spackle about the size of a quarter in your hand. Roll it into a ball, then go around and squish into each hole while immediately wiping flat with your finger. This sounds so basic, but if you do it this way you won't need to sand anything. Your finger is a better gauge of a smooth surface than any tool. If the holes are bigger, then use a putty knife

Spackle. You can purchase it at a Home Depot or Lowe's, it comes in a bucket. You'll need a large putty knife, to smooth the surface, and then you sand it, to make it smooth. This is a messy job, cover your floors. Wipe it down with a damp rag. Then your ready to paint.

For small nail holes, I've used a softened aspirin tablet and chalkboard chalk...on hand, cheap and effective.