Rivalries:

Most Iconic:

1. Scorpion v. Sub-Zero I / Noob Saibot
2. Sonya v. Kano
3. Kitana v. Mileena
4. Johnny Cage v. Goro
5. Liu Kang v. Shang Tsung

My Personal Favorites:

1. Sindel v. Shao Kahn
2. Sheeva v. Motaro
3. Cyrax v. Sektor
4. Fujin v. Raiden (from Fujin's biography in Mortal Kombat Armageddon)
5. Jade v. Tanya


Icons:

1. Scorpion
2. Sub-Zero II
3. Johnny Cage
4. Raiden
5. Sonya
6. Goro
7. Shang Tsung
8. Kitana
9. Jax
10. Liu Kang

As far as icons go, there is substantial proof Sub-Zero is more iconic (or certainly more liked) than Scorpion.

Poll 1

Over 67,000 people weighed in during 2004, and Sub-Zero easily won. You have to keep in mind a lot of people don't follow MK that closely, and even though this poll dealt with MK1 people voting for Sub-Zero almost surely were voting for the Sub-Zero we've seen since MK2.