Depends, they have so many albums and each one is very different. If you like melodic black metal, go with "memoria vetusta II". If you want scary industrial black metal, go with "the work which transforms god." Every album is quality in its own way.
Their music gets progressively more blackened/epic and less folky as time goes on. A perfect mix of black metal and folk instruments is on Verisäkeet, which is my favorite metal album of all time. Also, the vocals are amazing IMO. Here's the song Pimeä:
If you want more symphonic/folky metal, Kivenkantaja is the way to go: