There are things that can make certain chemicals lose their strength. Bleach will be affected by heat and sunlight as an example but granular chlorine should be pretty stabile. There are different strengths of chlorine and one may be more concentrated than the other. Some manufacturers also did things to their chemicals that could cause them to "work better" there was a product back when I got the tub that combined chlorine with non chlorine shock, it still maybe sold today.
As far as brands for chlorine I buy the stuff sold at RHTubs.com in the 5 lb tub (and I've had them for a few years now), I use Sea Klear non chlorine shock and store brand baking soda. I do keep a bottle of HTH PH down around in case I need it. I also keep Sea Klear clarifier around as well. For borax I will use the 20 mule team stuff but tend not to need to use it in my tub.