OK, so Lego's Star Destroyer (set 66211) is pretty dang good... however, I had to be picky with it. So here's my modded model. I rearranged and added a bunch of pieces to make this as movie-accurate as possible. Not only did I add to the surface of what was already there, but I actually built a complete custom underside. You see, this set, being made for playability, had no bottom detail. The underside was completely flat so that the model could be set on the floor or on a table for play. Well, I went in and finished off the bottom details and even gave it display stand legs for that truly professional look.
Reference pic from Star Wars: The Essential Guide to Vehicles and Vessels.
LOTS of added detail here. Though the over-all appearance hasn't changed much.
Close up on the command bridge. Added a ton of detail to the front. I also made the command bridge window smaller, as it used to span across the entire front.
Close up on the ion cannons. The original set only had two on each side, so I added two more for the full four on each side.
The original didn't have much detail on it's sides so I went in and added various bits of detail... including a small turret on each side (center of pic).
The back originally had very sparse detail. For some reason, Lego also added a bunch of bright red and blue pieces here, so those were replaced with grey ones. Along the back spine, I added all the horizontal "ribs" as well as a smooth black tile near the bottom, which is where the trash ejection port is supposed to be.
Back view. The original model had a fairly sparse and open back... you could actually look into the ship from the outside. Needless to say, I filled that completely in. I didn't have enough grey pieces to fill the entire back so I just continued with the black that was originally used. Might go in sometime and replace the black with grey... but not any time soon.
I also created a micro scale Millenium Falcon that can attach right to the back, just like in the movie!
Here's the now complete underside. LOTS of detail here. I managed to design the panels so that it would have not just the main hangar, but also the small front hangar. I also incorporated the shield generator towards the back. The front display stand attaches into the front hangar while the back two attach into openings at the back.