What are the best perennials for container displays? Thanks!

Salvia, hosta-big ones for taking center stage, minis for tucking along the edge, roses, sedums- both tall and groundcover, and I'll have to think of others.

What size container? What is the exposure (mostly sunny or shady?)

A general rule is to choose plants that are hardy to at least a zone lower when using them in pots. Being in Z6, you would want to choose plants hardy to Z5 (or lower).

I keep a lot of perennials in pots , mostly because I am indecisive and not for display.
I think the problem you may have with display perennials they don't flower all year like annuals.
But they do well with annuals

I have :
coral bells
ferns
Hosta

Penstemon
hardy geranium
sedum
coneflowers
spiderwort
lavender
dwarf iris
small daylilies
dianthus
brown-eye and black-eye susan's
there are many more, the above are currently in pots , overwintered in zone 5