Awesome idea! I think anything with the description "slow to establish" is worthy of consideration. These are the kinds of plants that most instant gratification-type gardeners will tend to avoid.

Some of my favorite connoisseur plants:

Dodecatheon meadia
Gillenia trifoliata
Orienpet/OT lilies
Martagon lilies (species and hybrids)
Dictamnus
Lycoris (squamigera, chinensis, sprengeri, longituba)
Cephalaria gigantea
Geranium pratense 'Violaceum Plenum'
Crambe maritima

All of the above can take 2-3 seasons to really start showing what they are capable of Also, nothing in my world says "class" better than naturalized minor bulbs.

The Itoh peonies are fabulous! I planted 'Bartzella' this spring but, of course, it hasn't done much yet ;-)

I had bad experiences with the term 'choice plant' used in various catalogues. With me it usually translates to 'ungrowable' for some reason.

What do you mean by unique? Nobody else has it in the radius of X kilometers? Or very rare for some unknown reason? For me even common plant grown to perfection can have a 'wow-factor', well grown big Dicentra spectabilis is one of the most beautiful plants in existence. A sickly rarity will not WOW me at all.

Some plants with WOW-potential in my eyes:

Delphinium Lanzentraeger
Aster glehnii Aglehnii
Iris sibirica Over in Gloryland
Tall varieties of Lilium Martagon
Helianthus Soleil d'Or
Eryngium Forncett Ultra
Lilium davidii,
Helianthus Triumphe de Gand
Hosta Manhattan, Sun Power
Well grown Campanula Sarastro

There are a couple hybrid lady slipper orchids that would wow me in a garden.... Mostly because they are so hard to please but also because they are so beautiful. But unless you have a ton of money and/or no guilt in looking at a dead plant, I wouldn't recommend trying them.

Maybe I'm just stuck on native wildflowers but double bloodroot and double trilliums also come to mind as choice.

Well grown delphiniums always wow me, the bluer the better.

It also depends on if you want to wow yourself, as an experienced gardener, or non-gardener friends. Innocent visitors to my yard are speechless at BIG things - the giant hostas and tall ferns, and especially the really tall plants - New York Ironweed (7 feet), lovage and Joe Pye (6 feet), cup plant (8 - 9 feet).

It seems to me that people who don't do full-contact gardening think of plants differently - when you say "flowering plant", they think of zinnias or impatiens, while you're thinking of Itoh peonies, so when you show them your peonies, you are opening up a whole new world to them.

I think that you need to have some plants that are easy to grow, even if you are specializing in ladyslippers, so if a visitor says "oh, I could never grow that", you can say "sure you can!", and give them a seedling, just to lead them down that slippery slope.