What's your pick? My picks are from Deathly Hallows (because I can't remember anything from other books):

#1. The Deathly Hallows chapter
What Fred said on Potterwatch:

"For instance, this new idea that You-Know-Who can kill people with a single glance from his eyes. That's a basilisk, listeners. One simple test: Check whether the thing that's glaring at you has got legs. If it has, it's safe to look into its eyes, although if it really is You-Know-Who, that's still likely to be the last thing you ever do.”
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#2. The Missing Mirror chapter
When the trio arrived at Hogsmeade, they were saved by Aberforth's Patronus, and Ron asked Aberforth if he sent the doe to help them find the Gryffindor sword. Aberforth said:

"Brains like that, you could be a Death Eater, son. Haven’t I just prove my Patronus is a goat?"