Which cover lifter?
Well it depends on your means and the application.
For a plain no nonsence lifter, I like the EZ lifter. Takes me about 15 minutes to install and is simple to operate. I does require 18" clearance behind the spa. Can be used on round spas with the optional lifter 'feet'.
I also like the Cover Valet. A little more complex to install, it requires very little clearance behind the spa and lifts the cover vertical, above the spa bartop so it can be used on spas sunken in a deck or that have a skirt-mounted gazebo installed. The lifter has gas-shock assists that make the removal one handed operation-- Do need to be wary though, gas shock lifters require regular maintenance and you need to be sure to 'unlock' the gas shock before you open the cover.