It's hard to pick a definitive Alton Ellis LP representative of this site.  So much of his work has been praised here and people's favorites are all over the map.  So, I'll go with MY favorite Ellis LP.  Mr Soul Of Jamaica is a compilation of his Treasure Isle era ('67-'69) released by the label in 1974 (there's a more recent CD that expands even more.)  This is pure soulful rocksteady reggae, plain and simple.  It features killer cuts  like "Willow Tree," "What Does it Take", his cover of "You Made Me So Very Happy", and so on.