What is the most useful and cost effective Anker product, whether it is the battery pack or selfie stick?
Would depend what your in the market for as to what you would buy.
Personally if I was on a very tight budget and was after a battery (for size and emergency phone top) up I would probably go for this
https://www.amazon.co.uk/dp/B00P8SY7HQ/ref=pdp_new_dp_review i just recommended this to another dude in another thread. i thought you might find it useful too. like i told him, i get to charge my iPhone 7 2.5 times before it dies out on me. its an incredible little device
If you are on a budget, I’d say the best product to buy would be whatever you have a need for.
If a different product, that you have no use for is on sale for $5, would you buy it? Yet if you needed a power pack and found for $15, perhaps that is what you truly need?
My apologies if I’ve misunderstood the question.