Choose a pitcher for lowest cost, a faucet filter for on-demand convenience, and countertop RO when broad dissolved-contaminant reduction justifies the higher price.
Pitcher filters
Pitchers require no installation and are the easiest to move. Their limitations are small capacity, slower throughput and frequent cartridges.
Certification scope varies widely, so a cheap pitcher should not be assumed to handle lead, PFAS or fluoride.
Faucet-mounted filters
Faucet-mounted filters trade portability for convenience: filtered water arrives directly from the tap. Compatibility is the biggest practical limitation.
Countertop systems
Countertop filters range from simple carbon systems to powered reverse-osmosis appliances. RO offers the broadest dissolved-contaminant potential, but costs more and produces reject water.
Use the contaminant to choose the category
If the goal is only chlorine taste, RO may be unnecessary. If fluoride, nitrate or broad dissolved contaminants are the reason for treatment, a certified RO system can be more appropriate.