I can't name one title that is the most, but I'll give you two that I think should be considered...incidentally, both games released by Parker Bros.
Reactor
Although simplified (what 2600 games aren't?) from the arcade game (which I love), I thought overall this was an excellent port. I've played the VCS version with a trackball for a truer to arcade experience (and I think it plays better with the trackball).
Star Wars: Return of the Jedi - Death Star Battle
This was a cartridge that sat in my storage tub for years and didn't see any play. One day I decided to pop this cartridge in, read the instruction manual and gave it a go. I found this game to be an excellent two-stage shooter.