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This question, as you know, is a very subjective question. You could ask 10 different people and more than likely you would get 10 different answers. However, I think for me personally, the earlier iterations of of the Impala are the “best”. As you stated in your question, the 1958 Impala was a game changer because it was the first version of the classic car. The ‘58 boasted chrome decoration with sculpted fenders, Chevy’s first dual headlamps, and triple tail-lamps. It was originally introduced as the top of the line Bel-Air with a few different engine options as well.
The ‘59 Impala is probably the most drastically different version of the car stylistically. That year it became it’s own standing model without the Bel Air tag. It was two inches longer in length, had a rear “bat-wing” lid and “cat eye” sideways teardrop tail lights that were only found on the 59. That year Chevy also added a four-dour hardtop and a four-door sedan. Take a look at some pictures of the ’59 online. They are a completely different car then the other versions of the Impala.
The 1961 Impala may be the most influential version (and possibly the answer to your question). That year Chevy introduced the SS (Super Sport) badge and the 409 engine, thus essentially making the first “muscle car”. The ’61 SS came with knock-off wheel covers, heavy-duty springs and shocks, metallic brake linings, a padded instrument panel, and a Sun 7,000-rpm tachometer. The SS came with either a 348 cubic inch V8 or the 409-cubic-inch V8 that could hit 60 mph in seven seconds and 360 hp.
I hope this answers your question or at least gives you some kind of background so that you can decide on which one you like best. As I stated before, I don’t think there is a wrong answer to this question but these are just my thoughts on it. Good luck on your project and again, thank you for consulting MyClassicGarage for your automotive history question.