Toppest is not a word in common usage: topmost or uppermost are recommended.
Currently, toppest is not defined in any of the major dictionaries, and while the meaning can definitely be understood - it'd be recommended to use one of the following:
Highest follows the form you were originally wanting - the first thing is high, the second thing is higher and so the thing that is most high - is highest.
This works for high as it is describes a scalar quantity. That is, you can be a certain amount of high - "very" high, "kinda high" etc. As such, you can compare how high two things are in relation to each other (one may be higher than another).
It unfortunately doesn't work for top which describes a non-comparable position. That is, you cannot be more top than something else - although one thing may be on top of another. This is the same for words such as best, where you cannot be the bestest as you cannot be more best than somebody else (one of you is better and so they are the best).
uppermost and topmost are synonyms, meaning:
(uppermost) situated in the highest or most prominent position
(topmost) highest of all
As these are absolute locations, they also can't be compared (you cannot have the topmostest, or uppormostest).
They are relatively common terms, making it clear that you are talking about the absolute top of something - such as:
The topmost branches of the Scottish Pine