By far and away my favourite link when thinking about the garden are the Thompson and Morgan websites.
http://plants.thompson-morgan.com/ for plants and http://seeds.thompson-morgan.com/uk/en/ for seeds. If I'm planning for the business I'll usually look at the commercial site too - http://wholesale.thompson-morgan.com/uk/en/?set-country=uk - for plug plants so that I can try to work out how much things cost and what they could be sold at. I have found Thompson and Morgan seeds to be the widest range and the most reliable giving me the highest percentage of seedlings so far. Overall I've tried Nicky's nurseries, Suttons, Unwins, Johnsons, Town and Country, Mr Fothergills and eBay.
I was, therefore, very pleased to see that an independent seed trial by Gardening Which agreed in the germination rate tests. This is T&M's own press release http://seeds.thompson-morgan.com/res/pdf/press/TMUK0810.33.All%20Round-1.pdf - but I also read the same in Gardening Monthly in their review of the Which trials.
Most disappointing of all the ones I've tried are eBay (various sellers) but that is mainly because of the lack of information that arrives with the different seeds compared to the information on the commercial seed packs.
That's not to say that T&M seeds give perfect results everytime but I think that this is more due to my lack of experience in applying the necessary conditions (wrong temperature usually - on top of the boiler in the outhouse just didn't work too well):
stuffing up the planting out (e.g. the pumpkins which have died everytime I have put them out)
the climate in NE Scotland (Sweet corn and french beans have never got higher than a few inches before the summer has finished and the cold has stunted them to face eventual death in autumn)
the damn slugs or rabbits have got to them because I haven't grown them to a decent maturity before shoving them in the ground in the main part of the garden (lupins - everytime the rabbits and or slugs get to any lupin that is under 2ft tall when I plant out first year seedlings)
rather than the seeds themselves. This is probably true for the other companies too - either not enough instructions or I haven't been rigorous enough - and, as I said - for ebay it is generally the lack of instructions that condemn them to death at my hands. So to sum - I recommend T&M seeds - and I haven't even had commercial sponsorship to say that - though i am open to bribes.
Plug plants - next post I think - JD Parker, T&M or Van Meuwens usually - and that is mainly because of the freebies - I am still a real sucker for giveaway and freebis - but that's another post.