From the Outlaw Jose Wales.
"Reckon so Josey Wales". My wife uses that line at work every day it seems. There was a bunch of great lines in that movie from a "Missouri boat ride" to "Are you gonna pull those pistols or whistle Dixie?" and all the others already mentioned. Plus:
Senator: "The war's over. Our side won the war. Now we must busy ourselves winning the peace. And Fletcher, there's an old saying: To the victors belong the spoils."
Fletcher: "There's another old saying, Senator: Don't piss down my back and tell me it's raining."
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Lone Watie: "I'm an Indian, all right; but here in the nation they call us the "civilized tribe". They call us "civilized" because we're easy to sneak up on. White men have been sneaking up on us for years."
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And another all time great although it wasn't Clint that said it:
Lone Watie: "Get ready, little lady. Hell is coming to breakfast."
There are a lot more actually. Too many to list.