I love the 4 albums, but without a doubt my favorite is A rush of blood to the head, also among my favourite albums ever. It's the best lyrically:
"Look at the stars, look how they shine for you, and everything you do, it was all yellow"
"Lights will guide you home, and ignite your bones and I will try to fix you"
"So I'm gonna buy a gun and start a way, if you can tell me something worth fighting for"
Of course those 2 ines above are great, but obviously this one is deeper, the lyrics to X and Y were kind of cliche (Of course some lines on A rush of blood to the head are, specially on tracks like warning sign or the scientist, but it's not a trademark) and on parachutes extremely simple and if you're not in the mood, cheesy.
Then musically, Parachutes is a terrific album full of that relaxing and raw sound, it's a great formula, but you know it can be repetitive and simple at times. X and Y is full of arrangements and it's kind of futuristic, but it was too cliché (Speed of Sound for example).
AROBTTH is more complex musically, The four of them do a great job, the album is much more diverse and although is not something revolutionary is a sound many bands have been trying to achieve. (Snow Patrol would kill for a love song like Green Eyes and Keane for the scientist just withouth the guitar).
And besides, the artwork to arobtth is the best