Keeping this objective as I can, but obviously influenced by my personal preferences and experiences…in no particular order:
Black Seas Barracuda: The “original” pirate ship and the basis for construction for every ship that followed. Imperial Flagship is awesome, but the BSB is where it all started; instantly recognizable and has aged well despite being three decades old.
Taj Mahal: Despite the recent release “from the vault” this set was recognized for years for being the largest and also having the highest piece count. Certainly and expensive set that many people couldn’t afford, but many people wanted just because of its size.
Green Grocer: While not first on the block within the Modular Buildings line, its release indicated that this theme was more than just the CC and it added interior features. Moreover, it’s just a beautiful building that has aged very well as is instantly recognizable within the Lego community. It could be re-released tomorrow without any changes from the original.
Death Star 10188: This set knocked everyone’s socks off for years…and is still on the shelves in its updated form today. While I personally don’t like the set, it has fulfilled the role of Lego meets iconic cinema location. Z’OMG Lego Death Star! Every kid out there thinks this set is cool and its immense size/features makes it an impressive addition to any collection. Will be instantly recognizable for years to come.
10179 UCS MF: This set was iconic on release day and has become legendary across media and fills an important chapter in both Lego and SW collecting history. Anytime a work college of mine would hear that I had a large Lego collection they would ask “do you have that giant MF?”
VW T1 Camper: Yep, this is certainly an iconic Lego set. I have two non-Lego collector friends that have one of these sets. It has become a staple in many adult collection and is obviously a great seller and ambassador for the Lego brand. Once it finally retires, it’s contribution will finally be realized.
Lego Castle 375: I’m stumped here as I have had the pleasure of owning a many Lego castles over the years and they all are charming in some way. Hard to decide which one is most iconic…especially with new licenced castles such as 71043 Hogwarts and the Disney Castle pushing the boundaries of what people are willing to pay for a plastic toy. Right now, I’d have to pick the original yellow castle set 375, instantly recognizable for a mile away and set the train in motion for all castles that followed.
10228 Haunted House: Released during the height of the “zombie” interest phase in pop culture, it’s one of those special sets that provided the brand with something different. A fantastic set with great proportions and traditional architecture highlights. Seems to be one of those sets that will forever be desirable and iconic.
Winter Village Toy Shop: While not a massive set, it captures the imagination and sense of holiday magic perfectly. Great proportions and instantly recognizable, one of those sets that set the conditions for the ultimate success and longevity of the Winter Village series.
6399 Airport Shuttle (monorail): The only set on this list that I haven’t owned and built, it has a legacy and lure that is undeniable. Every time there’s one for sale local in my area is scooped up in minutes. Certainly iconic across the Lego brand.
Bonus:
Simpsons’ House: I remember when this was announced by Lego, the internet went crazy. Perfect execution for such a recognizable scene in television. This set is timeless.
Emerald Night 10194: Personally a set I could never part with. Perfect in so many ways. All in all a beautiful set that demonstrates what can be achieved with Lego bricks. Able to be appreciated even by those who aren’t into trains.
UCS R2-D2: A set every SW collector must have in their collection; great build with nice features. Perfect combination of Lego and SW, a set that can be enjoyed by non-Lego enthusiasts.
Technic Porsche: Fantastic set with great features. Took risks and pushed the boundaries of what can be archived within the Technic line. I actually have this set on display in my office at work.