The biggest number

I have given you all an index card. You have one minute to write down the biggest number that you can (barring infinity) using any mathematical function that you know (addition, subtraction, exponentation, ackermann, Knuth's up arrow, etc). Go.

>>1
This is sortof a neat experiment, until you try to find out who wins. How do you intend to compare these numbers? Who's going to figure out which is biggest?