The tension was very high in this episode and I can honestly say that out of all 3 this was the only one that had me on the edge of my seat.
Kingdoms are at their knees, and the only two characters who can save the day are getting weaker and weaker. At this point everyone has come to know and love all of the characters, and the stakes have become tremendously high. The Return Of The King is the final chapter, and since it is the climax and resolution of the epic journey, it has a little more intensity and urgency than the previous installments.
Obviously, I'm aware of the fact that the Lord of the Rings trilogy is actually one giant movie, but since it was released in parts, that's how I'm judging them.