Sure, it's hard to measure the value of assets. But look at it this way: In the long run, every company dies, so assets always go to zero. What investors (in aggregate) are left with is the net sum of dividends paid out.
To answer "what is the most valuable company in history" I would look for the company which paid out the most in dividends before it expired. Stock price is largely a measure of which company has managed to attract the most suckers.
Don't know why people would downvote this. It's a pretty good idea of how to measure the value of a company. It's somewhat shy of the true metric which would be along the lines of 'how much better it made the world'. Market cap is appealing because it's trivial.
These definitions would also allow comparing non-standard corporations, like churches and states. It seems like this is a natural comparison - it's inevitable that big companies are ranked among comparable countries.
To answer "what is the most valuable company in history" I would look for the company which paid out the most in dividends before it expired. Stock price is largely a measure of which company has managed to attract the most suckers.