Puh, that's a tough question! Because they're all so different. But the last three albums have more in common than the first one in relation to the others. I'd like to help you, but it's hard to say. But you can't go wrong with any album, because the music is always great.
Start with Alive Again, thats the first one I got into, and is easily the most accessable, plus contains the godly Eternal.
You have to get all 4 of them for the story to make sense, but you might want to get breathing shadow last as its a little harder to get into, but still good.