Well, some people will have different recommendations when it comes to Graveland. It really depends what you want. If you want Graveland material when they were a full on raw black metal band, you can get these:
Epilogue
In the Glare of Burning Churches
Carpathian Wolves
The Celtic Winter
Impaler's Wolves (this is just two song EP with two songs from Carpathian Wolves given the epic touch)
If you want the more epic Graveland, get any of these:
Thousand Swords
Immortal Pride
Following the Voice of Blood
Creed of Iron
Raise Your Sword
Memory and Destiny
Graveland also has a split CD with the nazi band, Honor. Honor has four songs, Graveland has 3. The Graveland songs are excellent, but Honor fucking sucks. So it's a judgment call.
If I had to pick four:
Immortal Pride
Following the Voice of Blood
Thousand Swords
and either Creed of Iron or Raise Your Sword. CoI has 5 songs, RYS has 3, so you make the call.