Like others who've posted here, I have many favorites. A major motivation for singing for me is to be able to sing those lines where music and words seem to resonate together into something greater, where you really look forward to getting to that part of a song. As a historian, I can disuss the pro's and con's of the Easter rebellion, the strategic necessity, the tactical absurdities, etc. and never bat an eyelash. But the image of armed men marching not to pipes or drums but "the angelus bells o'er the Liffy swells" gets me every time, my eyes are brimming even now...
There's a line from one song I never sing though, that sticks with me and that only gets truer and truer:
Money doesn't talk, it swears - Bob Dylan