Contrary to many of the reviewers here, I hold the unpopular opinion that Bob's work on the Island label was his best. By the way, for all the Island bashers, it also happens to be the only label that paid Bob real money for his work, and was instrumental in bringing his music to the world (despite playing a role in breaking up the original Wailers). That being said, I believe Legend (Island) is the best collection of 14 Bob Marley songs in existence. For my friends who enjoy Legend, I tend to then recommend Legend II (Natural Mystic). If the thirst has not been quenched at this point, the next natural step is to spend a few bucks and pick up Songs of Freedom.
Songs of Freedom goes beyond Bob's Island recordings (>'73) back to his Ska roots with the Wailing Wailers in the early sixties. It gives us some good samples of the transformation Bob's (and the Wailers') music underwent from Ska, to Rock Steady, to their early work with Lee 'Scratch' Perry, and then to the big leagues with Island. In my opinon, there is no better overview of Bob Marley and the Wailers' music in existence.
If Songs of Freedom doesn't satisfy you, and I'm not saying it should, because there is a lot of good material not on Songs of Freedom, my favorites of the Island stuff are in order: Exodus, Survival, Natty Dread, Kaya, Uprising, Rastaman Vibration, Burnin', Catch A Fire, Confrontation, and if you want the full set, you add in the lives and compilations: Live, Talkin' Blues, Rebel Music, and Babylon by Bus. Pre '73 and Island, it is much more difficult to guarnatee quality, but it's a pretty good bet anything produced by Scratch is worth owning.