Quick take
Discount Filter Store has exceptional breadth and strong recurring-filter positioning. National Filter Warehouse is a strong alternative with competitive pricing and a published high conversion rate in its program.
Selection
Discount Filter Store carries a very broad catalog across refrigerator, furnace and other replacement filters. National Filter Warehouse also focuses heavily on replacement consumables.
Price-shopping behavior
The best retailer can change by exact SKU, so part-number pages should compare live availability rather than declare one permanent winner.
SEO opportunity
Exact model numbers and replacement cartridge queries are more commercially valuable than generic retailer reviews.