Why is this seen the weakest of the three??

It's actually the favorite for a number of people, my son included.

For Tolkien fans it's the least "Middle-earth-ish". Except for the Ents, it's primarily a standard Swords&Sorcery story, which is probably why a lot of people prefer it. There are fewer "Huh? What?" moments for people who haven't read the book. Also, Tolkien had more important things to write about and he had seen real war so the battles were not the main thing for him.

Also, the book was only divided into three parts for commercial reasons (and because of a post-war paper shortage), even though Tolkien called the sections "Book One", etc. The middle section was never meant to stand on its own and the filmmakers had to create a big ending by taking the battle at Helm's Deep (pretty much a skirmish on the book) and making a big deal out of it.

I agree on TT being "the bridge that really holds the three films together", an essential part of the bigger story. But FotR introduces us to an entire world of someone's imagination and RotK tells us about the passing of that world and the beginning of ours.

I think this film is great. There really is no weak link in the Lord of the Rings trilogy.

RIP
Jeff Hanneman
1964-2013

This whole film capture so much emotion it's my favorite one. Lots of the rings is the only movie I have seen 10 times in the theaters and this one is my favorite

For me personally, Faramir is my favorite character, and I hated what Jackson did to his character in this movie. That's for another post though.

I'd say the main reason people find this the weakest (which I would agree with) is the slow pace. A lot of the film slowly builds up to Helm's Deep. I also thought this film had the most filler. Yes, ROTK is longer, but a lot more happens.

The main problem I had with this movie revolving around Helm's Deep was the fact that having Saruman's death occur at the end of the film would've felt anticlimactic. IMO that should've been the climax of TTT. Of course, none of this would've mattered that much had Jackson not cut that scene at the start of ROTK.

Still a really good film, even if not as good as it's predecessor and successor.

A lot of "book firsters" do because it tinkers more with the overall story structure laid down by Tolkein in order to fashion a workable stand alone film. I would argue that in terms of theme and tone it's actually close to the books though.

There's also the fact that it's the slightest in terms of ground covered and story progress, it really is a bridge movie and one that takes place mainly in the drab world of men. Some people find this less interesting, others find it draws them in more with its realism and grittiness.

Personally I just find it the most fun and re watchable one as the story is already in progress and the characters are off having their own adventures and overcoming their greatest obstacles. It's like Empire Strikes Back in that sense. I also just love everything about the culture of Rohan, the characters, locations, music etc...

To be honest all 3 are movies you just sit down after a long period of stress, watch and still be mindblown every time you watch it, its understandable because 1 and 3 are considered many peoples favourite individual film of all time, because the first one has adventure and all the characters and the world comes together for the epic journey that takes place, and is the start of the trilogy, and the last one is the final film that goes all out and is extremely epic, well-made and brilliant in its story, pacing, visuals and characters and ends the trilogy perfectly, the two towers is amazing but is just the adventure continued with saving Rohan being the main story and setting up Gondor, even though that film is still so rewatchable and has a slew of good qualities including the Battle of helms deep and more middle-earth adventure

And people who want a perfect book adaptation find that this is very faithful but less so than the others

This was my least favourite movie of the three. I didn't like what they did to Faramir's character or the elves at Helms Deep. That was a filler, so was Aragorn floating down the river and some scenes with Treebeard walking around with Pippin and Merry but not really doing anything meaningful. It was unfortunate because if these fillers had been cut there would have been room for character development and other scenes that were more meaningful.