First get the 'The Jester Race/Black Ahs Inheritance' rerelease. The Jester Race is often regarded as the best In Flames release, and it comes in a package with the exceletnt EP 'Black Ash Inheritance', which has been out of print for several years. After that, Whoracle is more thrashy, and Lunar Strain and Subterranean (available only together now) are more like older heavier melodic death. After these, Clayman is also excellent, although it is a different style, it is enjoyed by many. Reroute to Remain is their most disputed release, it is a new style that you may or may not enjoy. Try downloading a few mp3s atthis point to see if you will like it. Colony is loved by some, and not by others. There are people who think it is IF's best work, but others think it is their worst, or second worst. It is best to buy it last.
Steve