What was the largest Lego set you've ever built and would you do it again?

My buddy got me R2D2 this year, Chewbacca definitely took longer than R2D2 will in build time.

42055 Bucket Wheel Excavator.
It is physically very large. Larger than basically all cats and a lot of dogs. My daughter was 3 1/2 when I got it and it wasn't much smaller than she was then.
I've assembled and disassembled it several times. I think it takes 8-10 hours to get it together and not much less to take it apart. You really have to follow the instructions in reverse to get it broken down, and the pins make my fingers quite sore if I don't take it in stages.
The linked review is correct that the mechanics somewhat overtax the single motor, but I find the design so impressive that it doesn't matter.
This was the last "flagship" Technic set I ever bought. We don't have space in the house for more plastic toys this size, and I don't love the fact that the huge ones tend to be app controlled now. But I think this was a pretty high note to go out on.

When I was a child I loved bionicles and worked all summer to raise $60 for this double set of dragon looking things. Better yet, you could combine them and make a hybrid monster. Somewhere in that process of creating the hybrid I realized it was too much work disassembling and reassembling and completely gave up.
- I had a friend from school who was into bionicles. It looked really cool, but I have to admit that I just simply didn't understand it

The millennium falcon, the big one. It took about 16 hours of build time for two people. I think the only set bigger at the time was the titanic. I’m actually not sure if the Eiffel tower has more pieces, or if it’s only physically bigger.

I bought the LEGO Rivendell set close to when it came out. It was a fun one to build, and I would do it again. Honestly, I'll probably need to disassemble and reassemble at least some of it whenever I move.
- ...came here to post the same: we have a lot of flagship lego sets but rivendell is an unparalleled masterpiece, so exceptional that it makes almost everything else seem unworthy of shelf space by comparison...
  
  I don't have a ton of sets, but I absolutely love Rivendell. It is beautiful to look at, has all of the fellowship, and has so many tiny details. Truly one of LEGO's best sets in my opinion.
- Lining up those roof tiles was pure torture. But a beautiful set.

My kid bought me a Back to the Future DeLorean for my birthday, about 2000 pieces.
Initially I thought it was kind of a mis-gift, something they would enjoy more than me since I hadn't built a set since they were small and needed my help, but I made it a point to crack it open instead of letting it sit and it turned out to be quite enjoyable.

I've done 2 of the technix cars 1000-2000 pieces. They are fun but expensive. They take up a lot of space when you are done. The Ferrari had a lot of loose pieces for the price. As others have mentioned, look into led kits before you build it.

Not really the biggest, but most complex: the ornithopter set. Tremendous fun, and some of the mechanics feel borderline lego-illegal.

Probably the AT-ST walker from Star Wars. That was cool since I hadn’t made anything that bit, but it was all grey, and therefore as memorable an experience as the last 9 star wars movies
Edit: 8 of the last 9. Rogue One was a great movie

I can't remember the piece size of the top of my head, but physically, the Raiders of the Lost Ark temple is my biggest and would happily build it again.
My only worry is that the rubber bands might perish at some point.
I enjoyed building the ECTO1 but the stickers were really annoying. There's a large number of stickers representing rust and they're all identical.
I was going to leave them off so the car would look pristine like the original film but I changed my mind.

Probably fort legoredo, I wouldn't mind doing it again, but I think some pieces got mixed with some other sets from the wild West theme. I think that set also doubled my Lego gun collection as well.

There is a ducati panigale sitting inside a box which I haven't even opened for 2 years... I am too scared. Was waiting for my kid to get older and then make with. Almost there.... ;)
- The Ducati looks like it has a lot of pieces I don't recognize from other sets. That's pretty unique.

SPACESHIIIIIP!
- Was this a certain Lego set, or did you freestyle this spaceship into existence? How many pieces do you think?
  
  I'm guessing OP was talking about set #10497 Galaxy Explorer with 1,254 pieces.

Titanic, and no. I looked at getting an LED kit for it, saw how much I'd have to disassemble and decided not to.
- That's 9090 pieces. You're a madman. How long did that take?
  
  Maybe two weeks of nightly building after work while watching TV.

I built the Apollo Lunar Lander not long ago. I enjoy the experience of putting together Lego kits so sure, I'd do that again. Or even a bigger one like the Saturn V.
The problem is what to do with these after I finish assembling them. Nicer kits have a good number of specialized parts so I don't want to take them apart. So they just sit on a shelf and collect dust.
- This one is really cool. I don't remember seeing it before. Pretty cool design and there's alot of detail in there. @Deconceptualist@lemm.ee i understand that. They're a lot of fun to build, but then how long do you keep them?

Blacksmith set a few years ago. I got it as a Christmas present and if I had money I’d do large sets more often because of the joy I felt building that one.
- @waterore@lemmy.world Was it this one?
  
  Yup such a wonderful build for me

The giant Hogwarts with 6020 pieces! It was a lot of fun over a week, but my fingers hurt when I was done so I probably wouldn’t do it again. The stained glass looks really cool though!
- I don't think the picture does it justice. Being 6,020 pieces, this thing has to be Big, am I right?
  
  It was! I kept it split framing my tv for a while

Sopwith Camel WWI plane
- I've never seen one of these before. The engine and the wings are really interesting to me.
  
  I think i got it around 2003. Yea the engine was a rotary and the cylinders actually rotated with the propeller!

Death star. I'd probably build it again. Taking it apart seems harder for some reason.
- How long did it take you to put this thing together. It's definitely one of the bigger ones.
  
  Memory is a little fuzzy as this was.. Over 10 years ago (oh god I'm old).. I think I did a couple hours a day over several days. Probably totaling 8 hours

Personally for me, it was the Galaxy Explorer. I helped my brother put together the Temple of The Golden Idol from Indiana Jones.
I've never really had a big Lego set, but I'd like to some day. I really want Rivendell and the D&D castle/dragon that came out.