Most people might say the 9x13, and while I do use those a lot, I have found my most used pieces to be the ceramic hot/cold tumblers.
I keep several sets of these in my freezer at all times, and when we get something cold to drink, we grab one of these. I have gotten so I can't drink milk, juice, water or soda, anything cold, without these. I love to add some ice, pour the cold drink in and let it sit for a minute. It makes the drink so cold and refreshing.
I think I put these to more extreme use than any other TT I own, as they are used, washed in hot water and placed back in the freezer over and over and over every day. I've only lost one, as I dropped it in the sink and chipped it. I still used it for a long time after, until it started to absorb water and craze. I have many other pieces of TT that didn't get nearly this kind of workout and they failed, so I don't know why these are so strong.
I don't use these for hot, as I don't think they keep hot very hot for very long, but love them for cold use.
What is your most used TT piece?