Writing a thesis acknowledgement: should I address the person as "you" or use 3rd person?

I'd go (and did go) with the 3rd person format, for a simple reason: It's not a letter to that one person, nor a (handwritten) dedication/signed copy of a thesis. Instead it is something that is printed and which is (hopefully) read by more than this one person.

So you talk to a broad audience about that person, hence 3rd person. Only if it were a handwritten, personal message in one particular copy, I would ever go with "you".

(Another exception could be a dedication. In contrast to the acknowledgements the dedication is more specific and -- usually -- to one person or specific group of people, like family. In this case 'you' might also make sense, because here you address them personally.)

In my thesis I made an acknowledgement infographics. The logic was simply that nobody wants to read a dry acknowledgement page, so the best way to thank them is to make people want to read the page. In fact my thesis was often taken out of the shelf in my PhD department solely to look at it.

EDIT: here it is.

I will offer a contrary opinion: thesis acknowledgements are generally yours with which to do whatever you wish, within reason. If you prefer second person, this is the one piece of scientific writing where you can definitely get away with it. If you want to write it in a sonnet, that's OK too.