I don't know what exactly counts as a "simple" song, so I'll just say probably all of them. I love DT's more straightforward songs, and while they may be "simpler" to perform, I find the songwriting behind them to often be more melodic, and concise and catchy, so in that regard I don't consider them simple.
Sticking with songs already mentioned in the thread, I'll say These Walls, Hollow Years, Anna Lee, Another Day, The Silent Man. As I said, basically all of them.