Nakamichi made a few models which could be considered "the best," whatever that means. So did Tandberg and Revox.
If I had to pick just one it would be the Nakamichi 1000ZXL Limited. Why? Because it was the best Nakamichi could make and it was a fantastically beautiful machine. Top notch in every category, it was the ultimate Nakamichi and, by default, the ultimate cassette deck.
Tandberg, Revox, and even Nakamichi fans might have a legitimate argument with that "best" moniker but even they must admit that there may have been a FEW decks that were the equal of the 1000ZXL, there were none better.
For one a little more down-to-earth affordable I would choose the Pioneer CTF-1250.
But if you want the ultimate analog recorder, nothing beats a good reel to reel deck. Even the absolutely best cassette decks ever made can only approach the quality of a good R2R.