Start anywhere apart from Back From The Dead.
Strongest albums are:
Slowly We Rot
Cause of Death
The End Complete (best selling death metal album)
World Demise (VERY strong album)
Thats the first 4 albums. Then jump forwards to Frozen In Time, and listen to the other two after those. You pretty much cant go wrong with any Obituary album. Dead is also a gooden, its live, but its VERY solid. But its got like all the major early songs on there.
I just became a fan of that band.