Who has the most modern fleet?

'Book a cruise with us. Recommend a cruise with us to all your customers, we have the most modern fleet in the world'.
You've got no idea how many times I've heard that from the different reps that come to visit us from the different cruise lines. The only problem is they always seem to have a different definition of what the most modern cruise fleet means.
I can see why so many cruise lines want to be able to advertise this on their promotions as it does sound good, 'The most modern fleet at sea'. But with so many different brands saying the same thing how do you define it?