What is the best under sink water filter?

Best overall under-sink water filter for most homes: Aquasana Claryum 3-Stage Max Flow. It is a strong everyday pick because it targets a broad set of common tap-water contaminants, keeps useful minerals, and has faster flow than many basic under-sink systems.

If you specifically want reverse osmosis for dissolved solids, lead, arsenic, fluoride, or very broad contaminant reduction, pick Waterdrop G3P800 or APEC ROES-50 instead. Reverse osmosis is more thorough, but it usually costs more, needs more space/maintenance, and wastes some water unless using a more efficient tankless design.

Quick buying rule: choose Aquasana Claryum for city water taste/odor and broad filtration without RO; choose Waterdrop G3P800 for high-performance tankless RO; choose APEC ROES-50 for a proven, simpler RO system at a usually lower cost; choose CuZn UC-200 if you mainly want long filter life for chlorinated municipal water.