Possibly a bit obvious.
iona but, I have geranium 'Rozanne' in a tall terracotta pot, where it has lived for over a decade. It could probably do with hauling out and dividing up but my pot is one of those which narrows towards the top, so removing plants is a nightmare. However, I noticed the first incipient signs of buds - expect to wait till June, buit then it will thrash around, flailing flowery, perpetually extending stems, until November. And if you add in 3 or 4 upright annual euphorbia oblongata (teeny little footprint), the combination and colours (luminous greens and blues, with distinctive foliage), this might be a doer in your pots
iona. Failing that, I find the shrubby salvias (s,greggii, microphylla, jamensis) are also fabulous pot dwellers (I have over a dozen, from wildly enthusiastic peachy s.jamensis 'Sierra San Antonio', to the neat and floriferous deep purple s,greggii "Nachtvlinder' and all colours inbetween except a clear yellow. All of these bloom for months on end, can be gifted or sold by the dozen because nothing is easier or speedier to propagate than a twiggy salvia. I can even do you some to order, if you like...My favourite is the more upright purple "Christine Yeo', followed by 'Stormy Pink'...but there are many, many to choose. I love them all, having slipped almost as far down the salvia rabbit hole than the yawning pit created by rose obsession.
Finally, a decent perennial wallflower (such as Bowle's Mauve', 'Winter Orchid', 'Apricot Twist' will look good in a pot for 8months of blooming (with judicious nipping out and a tiny bump of slow-realease fertiliser.