Personally, I'm always looking for the kind of tone you see in the first half of the first season of TOS, or the third season (and some of the second) of TNG.
Those are really the high points for me. I enjoy the rest of franchise a great deal, but overall those particular periods in Trek's history are certainly my favourite (though there are also some standouts scattered across the various shows and seasons). It's also one of the reasons I've always enjoyed Exeter; it captures that tone very well.
But as with all else, it's a purely subjective thing.