There are so many great varieties of tickseed/Coreopsis available, and so many new cultivars introduced each spring that you could update half your collection each year for many years without running out of new plants to try. However, you probably wouldn't be able to bear to toss out half of any collection.
All the species are very willing to grow, simple and beautiful. Many hybrids and varieties sold at garden centers can't be grown from seed but for each of those there are a half dozen tried and true varieties that can be seed grown. You could dedicate a whole garden just to growing Coreopsis species and varieties. You can't go wrong with long-time favorites 'Sunray' and 'Baby Sun'.
Flower color is the only big limitation in Coreopsis. Expect gold, pale yellow, and some red-orange from crosses with the annual C. tinctoria, but take claims of pink or red varieties with a grain of marketer's salt. Even the "pink" species, C. rosea is more salmon tone that true pink.
When you've accumulated your fill of the short, late spring and early summer blooming Coreopsis (early mouse-ear coreopsis, C. auriculata, lanceleaf coreopsis, C. lanceolata, the "pink" coreopsis, C. rosea, and threadleaf coreopsis, C. verticillata), look for the tall very late blooming C. tripteris.
Below, left: Tickseed Coreopsis (C. lanceolata). Below, right: The threadleaf Coreopsis (C. verticillata).
Lanceleaf Coreopsis (above, left) is early to bloom. We'll have cut it back and seen some second bloom before we see much flower from the annual blue salvia planted with it (lower left in the photo). Threadleaf Coreopsis (above, right in its pastel yellow form, 'Moonbeam') is a mainstay of the midsummer border.
'Moonbeam' threadleaf Coreopsis is popular for its low profile (15") and pastel flowers that are easy to combine with so many other colors. However, it's not so robust as most coreopsis. C. verticillata varieties 'Golden Showers' (taller than 'Moonbeam') and 'Zagreb' (12") are more vigorous growers.
Easiest, best perennials