I have always used Sensi Seeds, always been excellent. I also use Royal Queen Seeds. Last year I used Expert Seeds for the first time, very impressed. I had their Sweet Zombie as a freebie and that was really good and I did a run of their Critical Blue Autos outdoors and that was the best weed I have ever grown.
I have recently been hearing good and bad things about Discount Seeds, so I bought I a load. Delivery was quick and they don't use excessive packaging like a lot of the others, which I like. Time will tell if the seeds are any good.
The only bad experience I have had with seeds was from a vendor called The Vault, I bought a batch of Afgooey by Greenhouse Seeds. All of them were duds. In 20 years I have never had a seed fail to pop, so I can't see that the problem was anything I did. The Vault didn't want to know.
The thing with seed vendors is how they deal with problems. For example when I ordered from Expert Seeds I waited nearly a month and nothing came. So I emailed them and they apologised and sent me double the amount of seeds by express delivery. Every vendor will have problems for time to time, it's about how they deal with it that makes the difference.