I would, without a doubt, say that "March Into the Sea" is the definitive Pelican song....I would say it is the only song you need, chugging and morphing along at 20+ minutes...I would say all these things if I didnt think all the rest of their work was so freaking good as well...But if you want a jumping off point, this is the place to start. It has the best elements of both their earlier, crushing, chuggin work, and there newer acoustic, post-rock stuff. Granted they have always combined those styles to make the music they make, just in different measures as they have progressed. If you are sold on this song I would then go buy Australasia and The Fire In Our Throats...as they both have their fair share of crushing riffs, post rock solitude and dreamy soundscapes...
If you are not familiar with Pelican, here are some ideas for you to digest. All the comparison to Mogwai, Isis, and Jesu are right on. I can see where people are going comparing them to Godspeed You Black Emperor and Explosions in the Sky, as they take their time getting to places. But those two bands are their own complete beasts. I often think of the instrumental song "Orion" by Metallica on Master of Puppets...Imagine if that song were 10 minutes, or in this case, 20 minutes, complete with crushing distorted guitar passages, with fierce drums, and with beautiful dual guitar pieces, interwovem over chilled out bass and drum passages...now youre getting the point. I hope...