Definitely give Tall Poppy Syndrome and Bilateral a listen first. They're both the 'best' that Leprous have to offer and the ones with the most traditional prog metal type sound. Then any of the others will do.
I've been listening to The Congregation a lot lately and, I think I really like it. No, not as much as Bilateral or TPS, but way, way more than Coal. I like the weird, repetitive guitar parts more than the endless oohing and ahing of Coal. Rewind, The Price, Triumphant, Within My Fence, Slave, and Moon are all my favorite songs with Rewind the clear standout. The only song that doesn't do anything for me is Pixel. It's not that it's a bad song, it's just very unmemorable. I've listened to it 7 or 8 times and I still can't tell you what it sounds like.
While it's not perfect, I think Congregation is a pretty worthy entry into Leprous's discog. Trim some of the fat (Pixel, Red, and either Down or Lower), bring the runtime down to around to 50 or 55 minutes and I think the album would be a lot easier to digest.