Size of rockery? Which way does it face? What is the soil? (acid neutral or alkaline) How specialist do you want to be? Otherwise we are just guessing in the dark. :)

I have never had a rockery but if I had then two plants I would be going for if possible would Lewisia and Saxifrages because they are so pretty

Lewisia is very fussy about how and where, so if you have one do look it up before you plant it.

If you are purchasing a lot of plants take care to get ones that flower one after the other so you end up with colour through each spring/summer month. If you buy them all in flower now - from next month you may find it very dull. Goypeadia has lots to see including Phlox sublata, Pride of London, Dianthus, Campanulas, Erodiums, Lithodora and Saxifraga.

Climate is also a very important question, since that is a huge variable in the USA.