Give me odorous at sunrise a garden of beautiful flowers where I can walk undisturbed. ~Walt Whitman
It's still a bit too soon to start this list, but then again, I'm not exactly known for patience. Mostly it's just a list of things that I've had success with (so far). In general, I look for plants that have year-round interest i.e. bloom for long periods or have interesting form or seed heads during the non-blooming season. Drought, humidity, and salt tolerance are also pretty important. So I really like plants that are indestructible.
Coneflowers / Echinacea - while I don't think this is the most beautiful flower in the world, it's definitely one of the best performing flowers I've planted. It grows, but not uncontrollably. It tolerates clay, heat, overwatering, and getting piles of salty snow dumped on it. One drawback; it's expensive, so I tried growing some from seed. I've actually had a moderate level of success with the seeds. Out of 10 seedlings, three are currently flowering (and it's late October!).
Russian Sage / Perovskia - can't say enough about this plant. It flowers forever, well into October. It seems to go with anything. It looks good in winter. It looks great from close-up or far away. It is drought tolerant. However, the garden centers seem to never have it (probably for all the reasons I listed above). One caveat: we've gotten a ton of rain this summer and it is flopping over and very unmannerly. This plant needs room and preferably a drought.
Ornamental Grasses - I looove grass. Love that they look good year-round (well, maybe except for Spring after I've cut them back). Love the sound as they sway in the wind. Love that I don't have to water them. I've had great luck with various cultivars of Miscanthus and I'm trying out Calamagrostis). Large sizes can be pricey, buty the Miscanthus are pretty slow growers, so it might be worth it for a bigger plant.
Hydrangeas - I've always loved the look of these plants. They look good almost all year. They come in the most gorgeous colors (not necessarily electric blue), and they have nice foliage. They do need a bit of water and, in my experience, are slow to get started (not so many blooms, slowish growth), but are sooo worth it. Once they get going, they tolerate a lot of shade (actually they'll poop out with too much of mid-day sun). You can also get different cultivars that bloom successively, so you can almost always have a Hydrangea in bloom.
Lavender - My Lavender 'Munstead' has been a star of my garden from the start. It grows in full sun, in clay soil, in salty wind. With dead-heading, it still puts out blooms in late October (with nights below freezing). Also, Lavender is more of a small shrub in my garden. It does not die back to the ground over the Winter (at least it hasn't yet), so has lots of Winter interest too. So it has the best features of small shrubs; it doesn't get out of control, nice form, but blooms repeatedly. I've read that Lavender can be finnicky, especially if the ground heaves during the Winter, and Zone 6 is probably the edge of its hardiness zone, but I've had amazing luck so far.
Butterfly Bush - Buddleia's are invasive in some places, so I understand people's concerns about them. They also look best when they're dead headed pretty religously, but in my small urban garden, they are real show stoppers and INDESTRUBTIBLE. I mean, I've moved these suckers when it's 90 degress outside and they still come back. They grow pretty fast and they are bug/bee/butterfly magnets. They are also drought tolerant. I wouldn't recommend growing them in a wooded lot, near conservation land, but rather in an accessible corner where you can dead head. Personally, I like 'Nanho Blue' or the white or yellow cultivars rather than the 'Dark Knight' that I see everywhere.
Knockout Rose - The name is horrid, but there's a reason why these roses are bestsellers; lovely foliage, long bloom, bushy, but mannerly form, cold hardy, disease resistant, even shade tolerant, and their actually pretty inexpensive. I was always hesitant to try roses because of the horror stories of dousing them with fungicides, or having them die over the Winter or no blooms, or leggy growth, but my Knockout changed all that. Now I'm in love with (shrub) roses and plan to add more as the budget allows. So if you want Roses, but don't want heartbreak, the Knockout line might be the way to go (sorry if I sound like a commercial).
Pink Double Knockout in front of Ravenna Grass in the front garden.