Khan is easily the best Trek film ever. I watch it repeatedly. They brought in a real director, Nick Meyer, who actually went back to the original series and found a great nemesis for Kirk. Then he followed up by actually directing the cast and not letting them coast through the movie like it was a Trek episode. Khan proves that with good direction, the original cast--Shatner included--can actually act. Shatner gets a lot of flack for his acting in the original series, but he was great in Khan.
I was disappointed by Nemesis for the same reasons I've become somewhat disappointed with Star Trek in general: too much summer blockbuster garbage and not enough substance. That and the fact they lifted the plotline from Khan (the Captain must fight his arch nemesis to the death and a major cast member sacrifices himself to save the Enterprise).
Trek needs a long, long vacation. Look how long the concept was shelved between the original series and the first movie. They could bring back the Next Gen crew in 10 years; they'll still be young enough to make a movie. Certainly no older than the original crew was in Undiscovered Country.