Mine is "If you ever fall for a woman, make sure she's got balls"--Buck, Halo 3 ODST.
[Edited on 02.17.2011 11:06 AM PST]
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That one is pretty good. Another one I like is at the beginning of H3 right before you take control of the Chief. Arbiter:"We must move quickly, the brutes have our scent." SGT Johnson:"Then they must love the smell of green." I also like this one from the end of H3. It made me sad though. Master Chief:"Hang on, I'm getting you out of here." SGT Johnson:"No...You're not. (He then hands the Chief the holo-chip with Cortana in it) "Don't - don't let her go. Don't... ever let her go. (Coughs) Send me out... with a bang."