Sweet Seeds are sensational. Potent, productive and easy to grow stable genetics.
Barneys Farm are pretty good, so are Humboldt Seed Organization and Dinafem if you can still find any of their seeds, since they were both closed down about 2 years ago by the Spanish Feds. Yes, American by name but in reality an umbrella company of Spanish giant, Dinafem.
Seedsman would rate as my lowest.
For retailers, Pev Grow in Spain would be hard to beat for value, range and service.