I read that the best clone is the first ones cut off of a seed.
If by best you mean the most vigorous vegetative growth, that is correct. That's where I get my moms. You may have to veg a 5th gen clone slightly longer, but other than that I don't think it has any effect on yield.
I can't say this with any certainty with cannabis, but it's a fact that with berries you actually get a greater yield out of a 5th generation clone than a 1st. But that is more applicable to people growing several crops a year, rather than one huge crop.
I know by personal experience that if you clone a clone of a clone of a clone it is like a photo copy. If you photocopy something and then take a photocopy of the next photocopy and photocop the next photocopy and so forth, you eventually wont even be able to read what is on the paper. It is the same thing with clones. The more you clone the same strain the weaker it will get, not stronger.
Nope. That's not how it works. Cloning isn't photocopying. DNA doesn't deteriorate. The bud growing off a plant you've been cloning for 10 years can be just as good as the original. The exception to that being genetic diseases that are passed on through the cloning process. Those diseases are passed along through clones, but not through seeds.
It it's a perfectly healthy mom, the buds should be just as good as the original.
Vigor generally applies to vegetative growth rather than actual bud quality. High vigor isn't necessarily a good quality in every case. With high vigor you also get more stretching and node spacing that can be less desirable.
Basically, I'll take a health clone that's 10' tall over a healthy plant from seed that is 10' in most cases. I feel like clones fill out quicker. I only grow my outdoor from seed because I can get +10' tall plant on a more reliable basis that is less photosensitive. Also I can cut more vigorous moms which is more desirable for me since they will always been in a vegetative state.
For my indoor plants, clones are 10000x more desirable for me. They start off with more mature, closer, asymmetrical node spacing and their heightened light sensitivity will cause them to flower more predictably. I feel like I can manipulate the shape of a clone in a much shorter period of time than I can a plant from seed. That applies to situation where I'm going for lolly-popping clones as well as bushing them out.