Things that thrive in large pots

hello, i have two large wooden planters that did contain Box topiaries. Even though i fed/watered them regularly they never looked really healthy so decided to put in the garden instead. I now have two empty planters and wondered (before i buy) what would be better suited to the planters.

The planters are on the patio and couldn't be moved elsewhere as there's nowhere else to site them. I did wonder about Dwarf Syringa as i already have a mini ceanothus in a large terracota pot on the same site ???

We're very exposed to the elements and no shelter in the way of hedging or trees.

Would welcome any advice. Many thanks

shazza