Of the models I have built, GPM's DKM Bismark was detailed but not all that complicated. It was just large. It took me the better of a year to finish it.
Ken West's 1/12-scale Apollo CM was very detailed and complicated, but some of the complications came from fit problems and butt joints. I have immense respect for Ken, but he is not a fan of joiner strips.
Overall, the most detailed, complicated, and enjoyable kit I have built was Uhu02's Lunar Module. In a scale model contest at the EAA AirVenture Museum, it won an award for "Most Complex." At at IPMS Regional meet in Madison, WI, it took first place in "Real Space," beating out several excellent plastic models.
Honorable mention goes to Alin Osarik's 1/33 scale Merlin 45 Supermarine Spitfire engine. I built it in 1/16 scale and, even then, it was full of greeblies.
Of the models I have seen but not built, I would nominate the 1/96-scale paper model of the Apollo Launch tower and crawler kits from
EduCraft Diversions. At that scale, they will be close to five feet tall. If I had another lifetime in which to build . . .