All of those are great choices, especially Transatlanticism and What Sarah Said. To be honest, there are several others I could have chosen instead of We Looked Like Giants—Transatlanticism, Marching Bands of Manhattan, What Sarah Said, Stable Song, Grapevine Fires, St. Peter's Cathedral. But We Looked Like Giants was my first Death Cab song and still gets the slight nod as my favorite.
@Sebastian: If you haven't already, check out Transatlanticism and Plans—though I liked Kintsugi, I would say that those two are noticeably better albums, each with a handful of genuinely great songs and a ton of good ones.
As for Coldplay, Midnight is a favorite of mine that I acknowledge has nothing at all to do with Coldplay's main style, but I love the song a lot specifically for its unique atmosphere, and it would make my top five favorite songs of all time. My top five Coldplay songs are probably:
1. Midnight
2. Every Teardrop Is a Waterfall
3. A Rush of Blood to the Head
4. Viva La Vida
5. Fix You
An unconventional top 2, I know, but there's something about both of those songs that I really love. The next five would probably be Amsterdam, Death and All of His Friends, Hurts Like Heaven, True Love and The Scientist.