What is the best way to paint 5 gallon plastic buckets?

Use Rustoleum of Krylon spay paint for plastic

There are paints that you can for plastics, follow the instructions so that it sticks to the plastic.

Paint gun or spray paint specifically for plastics.

Spray paint. Rustoleum paint with primer. They have many colors and textures.

I use Rusto,rum spray paint.

You can get spray paint made for plastic. I would spray one outside and put it in a large box to do it (to limit the spray going everywhere), outside for the ventilation.

We do this all the time with Krylon - Our Walmart has better prices than the Rustoleum with Krylon brand having "25% larger spray cans" in every color for the same price. It has primer built in, so no need to prep or topcoat. Do it in about 7 minutes and done. We prefer gloss, seems sharper and seems like it holds up better, never dulls or chips. Ours have held up for years and years, and we do this with other plastic tubs and containers (in addition to the 5-gallon-buckets/lids) regularly. Love their color choices!