I don't know if these sets are iconic for Lego as a whole, but they are iconic for me. Some are from my childhood, some brought me out of my dark age.
1) Black Seas Barracuda - the first lego ship, still holds up today
2) Eldorado Fortress - Awesome looking fortress, very nice for the time, and I think it still looks great today.
3) Medieval Market Village - One of the sets that brought me out of my dark age, great to see more civilian castle sets.
4) 6990 Monorail Transport System - Made my Christmas one year, one of my prized sets and I still have it set up in my home 25 years later.
5) Black Monarch's Castle - One of the best of the older castles IMHO. Nice colors, breaks the mold from the grey castles.
6) Haunted House - Unique theme, great build techniques, awesome display piece.
7) An Unexpected Gathering - One of the most iconic locations in LotR and Hobbit in Lego form.
8 ) 10210 Imperial Flagship - Another set that took me out of my dark age. This thing is so detailed that most people don't realize it's Lego until they get up close to it.
9) Camouflaged Outpost - One of the quinessential forestmen sets.
10) UCS Falcon - I don't own it, but I've seen it in person and it is massive, detailed, and accurate. Shows what can be done with these little bricks and pieces.
Honorable mention:
9493 X-wing - There have been numerous variants done on this ship, but I think this one will stand out as a classic. Helps that it had a lifesize model built and displayed in times square.
Orthanc, Ewok Village, Modular Buildings... really any large Lego exclusive. Lego saw the emerging adult market and starts to create large, expensive, detailed sets to appeal to that audience.