In the book, Pellinor Fields by a long shot.
On film though, I gotta' say Helm's Deep was much better. Helm's Deep never dragged and while not as epic (though 10,000 vs. 350 is epic) with fewer weapons and creatures, it was more intense.
During the whole of Helm's Deep I was on the edge of my seat. I still am. After years of rewatching it still is suspensful. The action is top notch and the desperation and hum an element played up makes for a cinematic knock-out punch. While Pellinor Fields in retrospect suffers from what Peter Jackson's good (but heavily flawed) King Kong remake did....excess and ego driven waste.
It is too big and too long. It starts out suspensful but it feels like less is at stake because we never connect with the people of Mina's Tirth. The plight of those in Helm's Deep is like a shot to the gut but we never get to really see the people of Mina's Tirth except when Faramir goes off to "die." That's it. And it is SO LONG. It just doesn't gel nearly as well.
When I first saw it, it was good but I felt it dragged and needed to wrap up. Sure it has its highlights, like the flying heads, Gandalf recalling Heaven, the banters of the heroes, Rohan's charge, etc. but the sum parts outweigh the whole which drags. In rewatch value it can become downright boring if not monotonous. It probably is the weakest point of ROTK but it makes up about half the movie (ROTK is still my favorite of the 3 by the way, barely surpassing FOTR though now, which is probably the best made of the 3 by a long shot).
In the end Pellinor Fields is exhausting but Helm's Deep is exhilerating.