Best Under Sink Reverse Osmosis Systems:

Waterdrop G3 Reverse Osmosis System

• 8-stage filtration system
• Tankless design
• 2:1 pure-to-drain ratio
• Smart faucet for water quality monitoring
• "Waterdrop G3 Reverse Osmosis System, and it’s honestly amazing."
• "What I love most is the tankless design—saves so much space under the sink, and the smart faucet shows you the water quality and filter life."

APEC Roes-100

• High-quality product
• Quick support from customer service
• Reasonably priced replacement filters
• "APEC Roes-100 has been installed for a few months now. Seems to be a high-quality product backed by a support group that offered quick help to me."

iSpring

• Known for quality products
• Supportive customer service
• Offers upgrades
• "I have been very happy with iSpring support... They gave me a reasonably priced upgrade."

Express Water RO5DX Reverse Osmosis System

• Reasonable cost (~$125)
• Outstanding water quality
• Consistent and great taste
• "The water quality is outstanding. It is very consistent, and it tastes like bottled water straight from the tap."

Kinetico K5 Reverse Osmosis System

• "Checked the Ecowater and Aquasana and went with the K5."
• No complaints, great water quality
• Water is great

NU Aqua 7-stage RO system with Alkaline + UV

• Reliable with 7 stages of filtration
• Includes UV filter for microbe control
• Alkaline filter for adding back minerals and improving taste
• "NU Aqua 7-stage RO system with Alkaline + UV is recommended for the best quality."

NU Aqua Tankless with UV and Remineralizer

• Takes up less space
• Quick filter changes
• Comparable water quality to tank systems
• Includes UV and remineralizer
• "I personally went with the NU Aqua Tankless with UV and Remineralizer."

iSpring RCC7AK

• 6-stage filtration with reverse osmosis and alkaline remineralization
• "iSpring RCC7AK" is NSF certified
• Reverse osmosis and alkaline filter for improved taste

Pros and Cons of Different Systems:

Tank RO Systems:

• Advantages:
  • More affordable
  • Stores water for emergencies
  • No power supply needed
• Disadvantages:
  • Requires more space
  • Water may taste off if stored too long

Tankless RO Systems:

• Advantages:
  • Takes less space
  • Tastes fresher
  • Produces less wastewater
• Disadvantages:
  • Higher upfront investment
  • Needs electric power to operate
  • Cannot store water for emergencies

General Advice:

• Look for NSF/ANSI 401 certification
• Consider adding an activated charcoal filter after the RO system
• Replace filters regularly
• Consider whole house water filtering system

Other Tips and Recommendations:

• Consider performance and maintenance costs when choosing an RO system
• Choose a reputable brand with good customer support
• Install a water softener before the RO system if water is hard
• Get a whole house softener + RO system for overall water quality

These summarizations are based on discussions and opinions from Reddit users who have experience with different under sink reverse osmosis systems.