Where are your favorite places to order seed.
Would depend on the type of seeds you are looking for of course as the list for flower seeds would differ from the list for vegetable seeds.
Here are links to just a few of the previous discussions of this question:
Where to buy seeds
Good places to buy seeds
Good seed sources
Buying quality seeds
Also check out the Rate and Review Vendors forum here to see the most (and least) recommended sources.
There is another very popular gardening site that has extensive resources for finding suppliers for seeds and plants. If you google 'plant scout' or 'garden watchdog' you'll find the site. GardenWeb is apparently worried about the competition and so won't let us post a link to that site here. I like GardenWeb for discussions, but the other site has this one beat hands-down, IMO, in the reference and referral departments. Both sites have their advantages and are best used together.
As was alluded to earlier, there are probably well over 100 very good retail seed suppliers/sources out there. If you narrow it down to at least a category (vegetable seeds, seeds of perennials, seeds of annuals, native seeds, woody plant seeds, etc, etc), you might get some specific recommendations.
When you do find a potential source, review that source's ratings on Garden Watchdog before entrusting them with your order/money.
I order from Stokes seeds, Vesey's seeds, T+T seeds in Canada and they all ship to the States. With the exchange in the dollar now you might get a good deal.
I also order from Park seeds in the States because they have good prices and our dollar has been at par for a couple of years. It's dropping now so it depends on what the Canadian dollar does in the next year whether cross border shopping will be worthwhile next year.