How to get the maximum item in a nonnumerical list

I got a package_height_max property list, and converted it to a short list like below:

HeightList = list("1.6 MM" "1.2 MM" "1 MM" "2.4 MM" "1.5 MM"
    "0.55 MM" "0.7 MM" "3.85 MM" "2.6 MM" "0.1 MM"
    "0.8 MM" "2.1 MM" "2.61 MM" "11.75 MM" "11.7 MM"
    "7.25 MM" "1.1 MM" "1.3 MM" "10 MM" "11 MM"
    "1.27 MM" "6 MM" nil "13.5 MM")


How to get the maximum height value?

I can get each height value one by one, but can't treat with a list .
i.e.
car(parseString(nth(1 HeightList))) -->"1.2"

mapcar(lambda( (x) car(parseString(x))) AllHeight)
E- *Error* parseString: argument #1 should be either a string or a symbol (type template = "SS") - nil

Anybody share me the soultion? Thanks.