iPhone 8S has weak battery, what is the best replacement.

All batteries should be rated for the same capacity…there are no real differences between batteries. Now, some vendors will claim higher capacities but be wary of these claims as Li-ion battery science has not evolved that much.

There are lots of low quality batteries on the market. Sometimes they are re-packed cells, other times coming from batches that didn’t meet QC standards. Stick with a reputable vendor like iFixit and avoid low cost batteries on eBay and Amazon.