It's one of the Star Trek films I actually never tire of. I'm sure there are plot holes, but the characterisation is possibly better than in many actual TOS episodes. I have such great time with this one, it somehow doesn't matter the Genesis device is a silly idea or how the Enterprise is obviously going to be the closest ship to Regular One, despite having only just left Earth.
At the other end of the scale, Undiscovered Country might possibly be underrated. Not Search for Spock underrated, but fairly low these days. I mean I hardly ever hear its praises sung and its reputation has somehow taken a few knocks. Because of its definitely final voyage feel, unusual Shakepeare obsessed Klingons and for not being Wrath of Khan, I suppose.