We used to have a reverse osmosis system but maintenance was tough and water usage was considerable. After a lot of looking we decided on Aquasana under counter two stage water filter. I would prefer a third stage allowing for specialized filters to handle regional issues and especially the option to add UV to remove cryptosporidium. However, this was one of the easier systems to install and maintain.
Pros: Clean taste, Easy to install, Long lasting filter
Have been using Aquasana's under counter system for many years. Though it is a bit arduous to install, I am confident that the water quality is superior and worth the effort.