Spock’s death makes The Wrath of Khan unassailable in my opinion. No other scene in the original crew films comes close. There’s also the Genesis device, which is genuinely inspired writing.
The Journey Home is probably a close second, since 'thar be whales in ‘ere!’
After those two, it’s a toss-up, as I probably enjoy the rest of them about equally. The Final Frontier has the worst story and execution of the lot, but the best ideas and intentions so it gets a pass.