I know there are some newer hardy gardenia varieties out there. I'm hoping to hear experience talking about which ones really are hardier.

I had both 'Kleim's Hardy' and a smaller 'Chuck Hayes'; considering the CH was much younger it died no more quickly, maybe even a bit less quickly. But by the, what was it, 5 or 6th round of record low temps in early March, they were both dead. Both would have been fine with the first drop to 3F if it had been the only one, and the KH had almost never been injured before. I'll probably just replace at some point w/CH, since the flowers are a little showier and no less fragrant. But they are both still deserving shrubs in the DC area IMHO, dying once every 20 years is manageable for a shrub if you don't mind the replacement cost and aren't using it as a "foundation" shrub. (OMG honey, our house lost its foundations! We'll have to move!)

I think Chuck is my favorite of the hardy gardenia cultivars. I also like 'Frost Proof' (smaller leaves than species) and one called 'Summer Snow' ('Chuck Hayes' X 'Kleim's Hardy' hybrid with large double flowers) which is supposed to be hardy to Zone 6.

However, keeping it in a pot (even a large one) rather than in the ground will significantly reduce its hardiness. Zone ratings all operate under the assumption that the plant is in the ground. The ground provides a great deal of warmth during winter. In a pot, the roots will essentially be the same temp as the air. Therefore, I don't think I would try a hardy gardenia cultivar in a pot unless you can move the pot to a protected location during winter (e.g., a garage or basement).