When I started with Mastodon in late 2010, I started with Crack the Skye and fell in love. The concept, atmosphere, and balance of aggressive and melodic was the perfect cocktail of music for me. Then The Hunter came out and I liked that, but didn't really listen to it through. Now I like it, but it took awhile for it to click with me. Once More Round the Sun is pretty accessible, and for me intensely personal since I listened to it around the time of my mom's heart surgery, so the lyrics really hit me and eerily match up with things that happened around the time. I could go on with that....
Long story short, I'd start with Crack The Skye