The Heptones are one of the best Jamaican vocal groups.
They have recorded in several different styles for many of Jamaicas top producers, such as Coxone Dodd, Lee Perry, Joe Gibbs, Harry J and Joseph Hoo Kim.
I think you should start off with "On Top" and then continue with "Night Food", "Party Time" and the underrated "Good Life" (with Naggo Morris on lead vocals).
All great suggestions. Also "Heptones & Friends (Vol. 1 & 2)".
As a side note, I remember reading in Lloyd Bradley's book "Bass Culture" (a must read) that at the end of the rocksteady era (1967?) and well into the birth and development of reggae, Leroy Sibbles (Heptones' lead singer/songwriter) had a hand in nearly everything that came out of Studio One. Coxsone Dodd had made Leroy Sibbles the top producer at Studio One.
"The meaning of life: the best of the Heptones 1966-1976" is a very good compilation where are lot of killers of their songs, with it you can find your way easily...
My favorite album is "Better days".
If you happen to like the Studio 1 rocksteady "on Top" album, als check out "fattie fattie" and "ting a ling"... My studio 1 favourite though is Freedom Line (listening to the reggae sound it's one of the later records they made at studio 1).
As far as the Better Days album is concerned, check out the sound quality as I have heard some cd's which are inferior to the Third World pressing. Excellent Niney Hollness production.