That's a pretty loaded question. There are lots of opinions about who has the best seeds and breeders. Generally, Sensi Seeds has been going downhill for some time now. I've heard bad things from old heads about how Jack Herer, Mr. Nice, Hashplant and several other flagship strains are not what they once were. Some are saying that they've lost some of the original mother/father plants (more importantly, the fathers), while others are saying that Sensi outsources their seeds now, meaning that they have some breeders on the payroll that they pay to make their seeds for them. Which from a purely business standpoint is fine, but in our line of business that shit wont cut it. Btw, people say the same thing about Nirvana.
Here's a thread that kinda explains what I'm talking about.
http://www.mrnice.nl/forum/9-patients-forum/5935-neccessary-thread-about-sensi-seeds.html
If you want quality, consistency, potency and no BS marketing tactics I would have to point you to Mr. Nice Seeds first. Their main breeder Shantibaba is responsible for White Russina, Great White Shark, White Widow (and those are just the ones I can remember off hand), while the breeder he works with (Nevil) is responsible for Heavy Duty Fruity, Super Silver Haze aka Jack Herer aka Deisel, Nevil's Haze (and bringing any Haze to the market in general) Afghani#1 amongst others. In fact, Nevil used to be the owner of The Seed Bank who then sold it to Ben Dronkers who renamed it Sensi Seeds, so most of the flagship Sensi Seed strains were created or introduced fist by Nevil of Mr. Nice Seeds. Also, if it tickles your fancy, MNS is the only commercial company that 100% for sure has G13 genetics.
So my first answer will always be MNS, for the win.
http://www.mrnice.nl/dhtml/strains.php
After MNS, Sannie's Seeds is an awesome breeder who himself has some damn good breeders in their camp too. E$kob@r and Mortabel. You can't lose with any of the seeds you buy from Sannie's, and their customer service is the best of anyone I've ever had. Basically, if you order seeds from sannie's, you'll get your seeds. And you don't have to pay extra for some bullshit guarantee. They're another no bullshit, all for the grower type of breeders. I love sannie's.
http://www.sanniesshop.com/sannies-seeds/
If you're asking about who has the best seeds, you could go your whole life through the threads of every cannabis forum and not come up with one