Whitestorm's death at the end of The Darkest Hour turns many readers into a blubbering pile of snotty goo.
Hell, Darkstripe's death scene. Sure, he was a traitorous piece of scum that poisoned a kit, but him dying on the ground, saying that "It's all black... There's nothing left..." was just.... Sad. Then, of course, Graystripe ruins it by saying, "Well, that's one less traitor in the forest."
Yellowfang. Especially when she tells Fireheart that she wishes he was her son, instead of Brokentail.
This Troper teared up at the line Cloudtail (who'd pretty much been quite the Jerkass at times) said to Brightpaw telling her she was still beautiful to him, after she'd been viciously mauled by the dogs, and had been horrically scarred, losing an eye and an ear.
Cloudtail: Sleep well, Brightpaw. You're still beautiful. Come back to us. I don't know where you are now, but you have to come back.
Forest of Secrets: Silverstream's death. Despite my emotional breakdown, I somehow managed to keep reading... Up until the end where Graystripe leaves ThunderClan, at which point I broke down crying for about ten or fifteen minutes.