Would like a a tree or standard shrub that is evergreen and can live permanently in a 15 gallon pot. Flowering would be a bonus!
Suggestions please.

Go to your local nursery and ask them for the best choice in your area. Keeping a shrub in a 15 gallon pot outdoors can be very challenging because of the heat hitting the pot, cooks the soil and dries it out very quickly. However people do it all the time, you just have to watch it closely.

A draft arborvitae would be nice. And their tough plants.

What is the light direction? I would plant an azalea or dwarf rhododendron. You can't plant anything that gets to big as 15 gallons, in tree/shrub land is not that big.

Heat won't be an issue in that area.

Daisy it's south. Should have been a tad more specific in my request for suggestions. In my dreams it is a 3 ft. blooming ball on a 4 ft. trunk. The purpose is to screen off a neighbor's house.

Oh, there's a Russian River in Southern CA? I was thinking Guerneville.

The only thing I can think of that would suit your purpose is a Privet but, everyone on this forum is going to yell at me for making the suggestion. If you just wanted evergreen with attractive foliage, it might be easier. Then I would suggest something like Golden Euonymus. Almost anything can be trained as a topiary. Maybe find something you like and then ask us if we think it would be a viable option.

Yes Daisyl I live in Gueneville. The deck it's going on is south facing so it's sunny all day. A variegated willow std. would be perfect. However if you can find one it's expensive and I don't know if they can handle confinement to a 15 gal. Had the privet thought too. Then I could train morning glories up the trunk.

I think there's nothing wrong with privet as long as it's not the invasive one. I understand there are some very nice ones available. Everyone screams when you say privet, but I have learned recently that there are some that are very well behaved. Same with other types of plants. Honeysuckle, for instance.