Best iphone 8 battery replacement kit?

There is no such thing. Apple doesn't sell parts for iPhone 8. Anything you might buy online is not an authentic Apple Battery. And trying to change the battery in your iPhone yourself, is not something the average person wants to even try and could render the device unusable. If you want to replace the battery in your iPhone, schedule an appointment at your Apple Store Genius Bar or Apple Authorized Service Provider (BestBuy is in the US) and pay them to replace the battery in your iPhone.

This link will help you get started --> iPhone Battery Repair & Replacement - Apple Support