What you look while going to have a backpack or rucksack
Brand? Or affordability, price?

The brand will not do the work of a backpack or rucksack. It is just a name attached to it. The price won't do any work on the trail, either. What matters to me are:

- getting the right fit, so the pack is comfortable and doesn't create sore spots, which can ruin a trip in nothing flat.

- getting the right size, so it carries what I need and only what I need.

- getting the right quality of materials and workmanship, so the pack will not fail when I use it. Brand sometimes can stand as a proxy for this criterion, but with so many counterfeit products out there you can't just rely on a name. Nothing matters but the actual quality of the pack itself.

- getting all the above at the lightest weight and best price I can afford.

Fit, fit and fit. The pack has to fit your body (pack fit is almost as individual as shoe fit). The pack has to fit your gear. The pack has to fit you with your gear inside it--in other words, provide comfortable support for your gear. IMHO, nothing else matters. That's why it's generally recommended to buy your pack last, so you can fit the pack to you with your gear (including the equivalent in weight and bulk of a week's food) inside.

Yep, that pretty well sums it up. Stay with the brands reputable stores like REI or Backcountry Gear carry, and you'll be above the minimum level for acceptable quality - to that limited extent, brand can be a shortcut to narrow the field. Beyond that, fit is most important, then size and weight, then way behind comes features (like number of pockets, sleeping bag compartment, stow-and-go hiking stick holders, hydration sleeves, lids that convert to day packs, etc.) That's my own priority set; yours may be different. (The 60L pack I carry now is arguably 10 liters larger than I need, but it fits better than any 50L pack I've ever tried. I don't compress my sleeping bag, rain gear, or tent as much as I did with a 50L pack to fill the extra space - I've never felt the need to carry more stuff just because I had unused space.)

I stick with REI. I agree with the other posters, but let me say that price is a driver. I'm unemployed, old, and poor, and unfortunately price is really important (after a carefree life of caring more about the "best" pack).
So, that said, I keep an eye out for REI outlet specials, and in-store sales and returns. Great bargains there.
That info frames my search pattern. Once I find bargains, then, I buy based on fit (first and foremost), capacity (I have accumulated a day pack, a 60L pack, and a 90L pack, and use all for different purposes), workability (terrible word, but I mean it is constructed to be compatible with my camp and trail habits - pockets, dividers, external attaching gadgets etc.), and that's about it. Price will drive me to opt out on a pack, unfortunately, however, I will buy only if fit, capacity, and workability are correct. In my long life I have known the misery of hitting a long trail with my dream best-brand pack that didn't fit right. Don't do it.
My message is don't be afraid to let price be important, and realize with a little diligence and patience you can find the perfect backpack for your budget.

Brand isn't everything. I have a cheap Academy backpack I picked up at a local Sport Chalet (a CA chain store) that is surprisingly well made, but doesn't fit quite right for me. It's okay for day hiking in winter (full of clothes and overnight gear, if needed), but I wouldn't want to wear it for days at a time. It does fit a friend of mine who took it to India for 3 weeks and it came back none the worse for the wear. My regular pack is an old Kelty made from Spectra (not one of the famous white ones, mine is blue) and it is lighter and fits better. Got it second hand right here.

If you know what you want, look for a used one if money is an issue. There are bargains out there if you are patient, do your homework and don't get too excited if you are bidding on eBay. Craigslist may be better since the sellers are usually local.