Is the pot in the sun, or shade?

I have a Kalmia in ericaceous compost which flowers every year. I put annual plants round it.

If you wanted perennials, then most would grow in a large pot as long as you fed them. Penstemons look good, remembering that they might need winter protection in colder areas. They have a very long flowering period.

If its fairly shady, I'd choose a variegated leaf Pieris - evergreen, flowers, new growth is red, looks good all year round. Needs acid compost though. I'd choose an evergreen with interesting leaves simply because any shrub only flowers for 2/4 weeks max - that means for 48 weeks of the year, you're looking at just the foliage.

Potentillas flower all summer - I've never seen one grown in a pot but would think it would work OK as long as it's repotted as it grows.