I disagree. Some Sigma lenses are generally accepted to be the best mass market choice at their focal length / price point. I think it's true to say that none of the mass market makers make the best at every focal length and price point so choosing the best would mean mixing Canon/Nikon/Whoever and third party options.
Let me put your chopped quote of my earlier post into context by leaving my qualifying statement after the chopped bit where it should (?needs to?) be.
Well, possibly not as ultimately good as their Canikon equivalents but good copies (Sigma have a bit of a reputation for sketchy QC - not a problem I've encountered with any of the 5 or 6 I've owned or own now)
come pretty damn close and in most cases, for a lot less money - a serious consideration for most of us. IMO, good copies of Sigmas do match Canikon's offerings in terms of VFM but it can't be denied that in terms of ultimate build and IQ terms, the Canikons (well, the Nikons certainly - unfamiliar with Canon IQ and build quality) are generally better but significantly more expensive. The amateurs among us are going to get much better value for money from Sigmas than we are from Canikon (providing we get good copies) and I know that by going the Sigma route for around 1/2 my bagful, I've saved a few thousand quid - enough for a couple of very nice fortnights in Crete. It also can't be denied that people have had problems with Sigma's QC - soft and misaligned copies do exist but I've also heard reports of similar problems with main manufacturers' offerings, albeit fewer than Sigma.
TBH, I could fairly easily afford to upgrade the Sigmas in the bag (that have real eqivalents in the Nikkor range ) with the Nikkors but at a significant cost. The only real benefit (apart from the cachet of the Nikkor badges over the Sigma ones!) TO ME would be the f/2.8 on the 14-24 but at the loss of 2mm at the wide end - not much in terms of mm but around 16% in terms of width! The cost (ignoring any trade ins)? 14-24 f/2.8, £1337; 24-70 f/2.8, £1229; 70-200 f/2.8 (not always in the bag but would be upgraded at the same time), £1589. The 8mm fisheye has no AF competitors - certainly none in the Nikkor range, with the closest Nikkor being the 10.5mm fisheye but that's Dx and rectangular rather than (my preferred) circular. That's a total cost of a bit over £4000 and at a guess, the trade in value of what I have is roughly £200 for the 24-70 f/2.8, can't find a 12-24 Sigma 2nd hand, so would guess at 1/2 new cost, so around £350, maybe and for the 70-200 f/2.8, around £200-£250. Total, around £800 or so - far too much price differential for my wallet!