I just came across our 2011 Ozformers calendar yesterday, and it reminded me of this topic... because the 12 characters we used on the calendar was based on similar vote we did four years ago.
The winning characters back then in random order... before TFPrime, TF3 and TF4 had any influence (damn, a lot has happened in just four years)...
- Optimus Prime
- Prowl
- Starscream
- Bumblebee
- Ironhide
- Soundwave
- Grimlock
- Hot/Rodimus
- Shockwave
- Jazz
- Jetfire
- Megatron

Later I want to do a cursory count of the toys released by Hasbro and TakaraTomy as these and other major characters, to see which ones really are the most iconic to have the most amount of toys.
Optimus is likely to have the most, and due to the recent movies and the last two cartoons (Prime and Animated) Bumblebee is likely to be second.
Then maybe Megatron and Starscream... but fifth onwards will be fascinating to find out.