I tried a variety pack from, ahem, another retailer first which allowed me to contrast the various blades out there. I chose Derby as best for me but would recommend anyone else does the same kind of comparison as the choice can be quite personal.
Why was this one right for me? In short because it had the best balance between Quality, Sharpness, Longevity and Smoothness of shave.
In detail:
Quality: 9/10 (very consistent)
Sharpness: 8/10 (easily sharp enough)
Longevity: 9/10 (I get a good 6-8 shaves before quality degrades)
Smoothness: 8/10 (better than a Mach 3)
In any single category you'll be able to find a blade that's slightly better, but I couldn't find one that was it's equal overall.
Oh, and at under £10 for 100 blades you can't get better value.