Hmmm. That's an interesting one. Looking forward to seeing what people say.
I can quickly name the 5 most over-used in my opinion : Firepower Nandina, Knock-out Rose, Leyland Cypress, Japanese Holly, Stella D'Oro Daylily.
grass would be at the top of my list. My neighbor hood is all native landscape...
Under-used, Yellowwod tree (Cladrastis kentuckea), poet's laurel (Danae racemosa), 'Autumn Minaret' daylily, old-fashioned roses like
'Zephirine Drouhin' and the native cross vine, (Bignonia capreolata) of course this answer is different for different parts of the country, but these plants are under-used in my opinion, also viburnums in general are not used much. Over-used , #1 Leyland Cypress, burford holly, crape myrtles, mondo, knock out roses.
For my area the over used plants are anything evergreen, Rhododendrons, Forsythia, and Hostas ...all beautiful garden plants, but often just thrown in without much thought to them because they're easy here. And I agree with Stella D'Oro daylilies .... they are everywhere as are rudbeckias, daffodils and lamb's ears.
Under used, again for my area, not including bulbs that have to be removed in the fall here, because ALL of those are underused.... would have to be Lavender, Clematis (people here often opt for morning glories), Japanese maples, sambucus, and Rodgersia
Over used in omaha NE: Knock Out Roses, Karl Forester Grass, Autumn Brilliance Maple, and the Stella De Oro Daylily and the last would be a toss up between River Birches and Burning Bushes.
Under used in Omaha NE: Oaks-any Oak, but the native Chinkapin is one of my favorites. Ginko's (there is a shrub version of this out now), Any rose besides the Knockouts and Nearly wilds... I love Red Cascade. Plumbago Ceratostigma-awesome perennial I've been using for years, sorry I do not know of a common name... and Fothergilla.
In northern Ca. Agapanthus lily,,Nandina, Photinia, Fortnight lily, Texas privet and any kind of a palm tree. Palms are not native to No Ca.
Yucca, vinca, burning bush, asters, and taxus. ( I am in Ohio)