Marigold33,
Campanula (Camp) and many others on this forum certainly know more about more Campanula than I do, but I do know which commonly available ones have worked in my gardens here. It would also be informative to know what campanulas have worked well for other gardeners and why they might recommend them to others.
Some may find the giant and the milky bellflowers too large, requiring staking, etc.. Various members here have mentioned small and medium sized Campanula as doing well for them. For instance, I also find the hybrid Campanula 'Sarastro' attractive and very well behaved, though not that vigorous. Several other medium-sized Campanula are quite showy, but terrible runners (namely Korean but especially spotted bellflower). In that size-range, I've found nettle-leaved campanula (C. trachelium) very dependable, quite nice and find that any seeding around it does is easy to control.
There is, of course, a large number of different herbaceous and alpine garden perennials that can do very well (given appropriate growing conditions). You mention balloon flowers. I don't use the dwarf ones, but commonly available normal-sized cultivars can be very long-lived here (taprooted (deep-rooted) perennials like that do tend to live a longer time). For me, the double-flowered (Hakone) series is particularly pretty. Like others, I've found the blue and white Sentimentals live longer than the pink ones.
I haven't found delphinium very long-lived in my gardens (2 or 3 years), but they're extremely showy.
Below Campanula latifolia 'Brantwood', a giant bellflower (June 12, 2015). I've had one 'Brantwood' for up to 10 years.
For what it's worth, I'd say gardening with mixed perennials is more challenging that gardening with annuals. With annuals, it seems to me that you can clear it all away at the end of the growing season (and if you have avoided bad seeders) you can then start with a clean slate next year.
Maybe have some carefully chosen perennials and some annuals. Or rather, you can avoid the tribulations of both by using shrubs!
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