As you can see from the responses it all depends on who you ask doesn't it? I like the looks of the '65 and the '67 best. The '70 model has often been referred to as the best looking Chevelle but then we are back to "depends on who you ask". Right now the 70 Chevelle with the LS6 is the king at the auction block. Guess that's the best way to decide isn't it...dollars and cents?
70 Chevelle SS LS7 TH400---yes there was an LS7 that was even more of a monster than the LS6....estimated 500 HP. I helped a friend rebuild one back in the 80's and it won several 1st place trophies. Had all the options too like A/C, power seat back releases, Vigilite system, Cowl Induction and more. Wish he had kept that one---one of a kind.
My personal favorite is the '65. BEAUTIFUL body lines that even today don't look old. Also it was the last of the engines without any smog pumps or other devices. Only a pcv valve. In '66, the smog (air) pumps were installed.