Well which will i start?

First things first: listen to the song "Hades" by Kalmah.
I think "Hades" says-it-all about Kalmah in a way if you get me?

You like "Hades"? Then Google-up "Kalmah Discography" and take it from there! :D
 
Like some already said... start with Swamplord and see how they evolved through every release.

I discovered Kalmah before TWL came out and they amazed me everytime with each new release. The further release all had something more without neglecting the main Kalmah sound.

Swamplord is the essence of Kalmah, lots of classics on this one indeed : Evil In You, Withering Away, Heritance Of Berija and the masterpiece Hades.
 
Always start from the start, if you don't like it keep going up until you find a period in their career which you do like, i find that is the best way to get into a band.