Milk. And this is my logic:
In my formula of eating cereal, milk is the constant, cereal is the variant. I've never ate and had cereal left over and needed more milk. It's always that milk is left in the bowl and I need to add more cereal in so I don't waste the milk. So to measure how much I want to eat, the amount of milk becomes my measurable constant.
Secondly, usually take my cereal to eat at another room - to watch a DVD or start gaming. And out of the two, milk has to be put back in the fridge, but not the cereal. So by the time I leave the kitchen, I'd like to be done with the milk. And I take the cereal with me so that I can add more into the bowl to make sure no milk is left too.
I'm a very stubborn person who likes to do the same things the same way (a bit like Sheldon from Big Bang Theory), and I actually can't think of a time when I added cereal first in my life...