I have a 67 RS, a 68, and a 70 RS. I have a hard time choosing between the 67 and 68 years as far as which is my favorite, but I lean towards the 68 because I have trouble getting the 67 vent wings to seal properly.
Between 67-8 and 69, I don't see enough of a difference to pay the price premium, but that's my own subjective opinion.
The stock 70 is a MUCH better ride than the stock 1st gens (all that weight is good for something!), but I don't like the lines nearly as much. Front and especially back, yes, but not the side profile. My wife calls it "The Poncherello Camaro". :(
My 70 is probably the most eye catching color of all my cars, but my 68 gets the most looks. Draw your own conclusion.
-John