I have two clay planters attached to the walls of my patio. I have tried several different plantings through the years and the most successful is Algerian Ivy (Hedra Canariensis). It is a perennial in zones 8 - 10 and an annual elsewhere. This is obviously considered a "spiller" in your container plantings.
I prefer the variegated Algerian Ivy because in the partial shade under the pergola it has a glow about it. I have learned to buy the larger plants and also to purchase two for each container. Many years I would try to get by with one in each planter because they are expensive, just didn't work, was not full enough to make a statement.
This is an unusual ivy because it is so large. The leaves are at least 3" across. It is very effective in containers with multiple plantings as it trails down the sides. The varieties are in a solid green and green with a white rim. I prefer the variegated but both are attractive.