Top 3 (in no particular order):
Territorial: http://www.territorialseed.com/ Excellent service.
Catalog is also good reference material. Never a problem with
germination. Prices are a bit higher than some (but not as high as
Burpee), but found to be worth it.
Nichols: http://www.nicholsgardennursery.com/ Good prices. A bit
more limited selection, but offers other items as well. One of the
best garlic suppliers I know of.
Fedco: http://www.fedcoseeds.com/ Coop. Good selection and several
varieties not found anywhere else. Prices are also very good.
Other I've tried:
Used to order a fair bit from Pinetree.. Good prices, good service,
but their germination didn't seem to be as good others. I'll still
use them for bulbs and tubers. Best seed potato prices I know of.
I've also ordered from Vermont Bean, Totally Tomatoes, Seymour's
Selected Seeds, Jung's... Which I now realize are all affiliated
(http://davesgarden.com/products/gwd/c/106 /). Products/service was
fair, not up to par with what I'm used to.
I've also ordered from Burpee and some of the other "glossy" catalog
providers. Good product, just too expensive when compared to others.