Well... here are the answers.

This is only a rough result, but since I don't focus on or discriminate against certain characters, the stats from my own collection (which is a fairly decent sample size at 6,515 toys and figurines from Hasbro and TakaraTomy), should be pretty close to the percentages you would end up with if you counted all Transformers toys ever made. (the bigger an unbiased sample size is, the more accurate its results would be to the entire populace)

Of the most iconic character names to have existed in toy form since the first two years of Transformers, these were the six that I thought would be the most prolific - Optimus/Convoy, Megatron, Starscream, Bumblebee/Bumble, Soundwave, Grimlock.

Their percentages in my collection (and total number in brackets), in descending order...

8.92% - Optimus/Convoy (581)
5.76% - Bumblebee/Bumble (375)
3.59% - Megatron (234)
2.52% - Starscream (164)
1.83% - Grimlock (119)
1.12% - Soundwave (73)

That would mean, that if you had all of the different Transformers toys ever made, there would be about 9% being some sort of Optimus... and about 6% being some sort of Bumblebee.
That also means, just those six character names, have given us almost 25% of the entire history of Transformers toys.

I was surprised that Bumblebee wasn't yet in the lead after the last 10 years of being the face of Movies and cartoons... but going through each year, 2009 and 2010 were the only years (so far) that had more Bumblebee toys than Optimus toys. Every other year since Gen1 (which only had 3 toys of each character name), it appears that Optimus had more, giving him an unbeatable 200 toy lead (thanks to 11 years of almost no Bumblebee toys, from Gen2 in 1995 to Classics in 2006). But if future toylines have a similar ratio to this year's Studio Series, which has 30% Bumblebee's and only one Optimus to the 7 Bumblebee toys, it might only take a few years for him to catch up.
Actually... if you counted Bumblebee's "stand in" during that 11 year gap (the role of a young, bright yellow character who looks up to the rest in his team, and tries to earn their respect and admiration - ie, Cheetor and Hotshot), if both of those had been called Bumblebee for taking on his "role" during the Beast Era and the Unicron Trilogy, he would have easily been number one.