I think I was in love with the first book mostly, because of the Jake&Roland odd touching relationship, it was just one of the most moving things I've ever read in all my life, and I had all Martin's last speech memorized, just fucking GENIUS.
And the second book was super awesome because of the doors, and the creepy woman of the many personalities that merged into Susan and Eddie fucking Dean which is a MONSTER OF PATHETIC NEEDY AWESOMENESS and I think if I had known what slash was when I read DT the first time I would have slashed Roland/Eddie like WOA.
And the 3th book was the one with the Mono and the riddles and the vectors' guardians (I think) and Eddie's development and it was SO cool I'm getting goosebumps.
And then obviously the 4th was the one with the flashbacks and Cort and David (the hawk) and the chilling fight, and Roland who isn't good at solving riddles because his mind is too direct and slow but relentless and STUFF OMG I'M SORRY I KNOW THESE BOOKS ALL BY HEART. *weeping in joy*