'T'

In my case, it's definitely "W". No question.

I'd say it would be the Spacebar.

Spacebar makes sense to me too... for me, at least.

ctrl, spacebar, del

I think the most depressed key is the "down" arrow

What a knee-slapper :P

Spacebar is interesting. In order for it to be the most commonly used (in a block of text, at least), no given letter can appear more times than the number of words in that block of text. I wonder how often that's true, on average? Intriguing.

Just looking at the above, I'm thinking you might be right.