They're all fantastic.
My favorite will always be "The stolen throne", the first book. The book was published even before DA:O and tells the story of King Maric the saviour (DA:O spoiler). The goal of the book was to cover the whole Dragon Age basic lore, and it accomplishes it with an action driven story that spans across all Ferelden along with a spectacular cast (including the most badass young version of Loghain Mac Tir, which in the book is basically God).
"The calling" gave me claustrophobia. Too. Much. Deep. Roads.
If you really want to understand what's really going on in "Awakening" read it, I don't really like it but I may be biased (I don't like the deep roads as a location). The cast include a young and unbelievably stupid Duncan and a young and incredibly racist Fiona.
"Asunder" gives me the vibe of a missed opportunity. It's nice, decent-paced and has a stellar cast (Wynne being one of the main characters, story somehow focused on Cole and something more than a Cameo for Leliana, Shale and Fiona) but at the end of the day it covers the events that led to the templar-mage war while it could've covered a much bigger story.
"The masked empire" is awesome. Each member of its cast is incredibly characterized. The story is much deeper than just the orlesian civil war.
Plus there's Ser Michel de Chevin. Ser Michel de Chevin is love. Ser Michel de Chevin is life.
Side note: "Asunder" and "The masked empire" take place in the same time span, and there are some small cross-reference in the two books. Nothing major or that you need to worry about.
The comics are meh. They're not canon (the main character is King Alistair) and pretty short. They make a nice read but only for the right price, last time I checked they were sold at an extortionate price.
Just for completition sake: "Dawn of the seeker" (the japanese animation movie about Cassandra) is a let down. I like to think the story is being narrated by a drunk Varric because it's pretty inaccurate and Cassandra is unbelievably over-powered (even Cassandra in Inquisition makes a hint about this).
TL;DR: read them all, possibly in chronological order. You won't regret a single second spent on them (unless you're like me and hate the deep roads).