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
topmost
uppermost
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
61urban dictionary is not a resource to rely on for anything in learning a language. it is mainly a joke site with joke or rude meanings. Never open a urban dictionary definition in the office. – WendyG – 2018-05-24T13:42:31.637
13Urban Dictionary should only be used for slang terms. Trust the real dictionaries for formal and proper language. The edit in your post is correctly changed to uppermost for what you wanted to say. Thesaurus.com is linked with Dictionary.com and the Dictionary side does not include "Toppest". The thesaurus is just trying to be helpful. – Jay A. Little – 2018-05-24T13:53:29.280
1There is the word topmost. And we do speak of "the toppest trees" , for example, meaning the tallest, and "the toppest twigs" which are the twigs at the top of the tree. – Tᴚoɯɐuo – 2018-05-24T13:57:42.373
13@Tᴚoɯɐuo you might but my teacher would have given me stern words for that. We talk of the "tallest" tress and the topmost or uppermost branches – WendyG – 2018-05-24T14:08:33.347
1Well, @WendyG, I don't doubt you. But let me give you a few stern words myself: you can easily determine whether I am right by consulting a couple of decent dictionaries. – Tᴚoɯɐuo – 2018-05-24T14:45:26.087
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@Tᴚoɯɐuo: I've never come across toppest trees before, and it doesn't figure in NGrams. There's *one* written instance in Google Books, but it looks more like a facetious "nonce coinage" (not even in the tip-toppest branch of the tip- toppest trees) from a semi-literate writer.
– FumbleFingers Reinstate Monica – 2018-05-24T14:48:08.1872“When I use a word,” Humpty Dumpty said, in rather a scornful tone, “it means just what I choose it to mean—neither more nor less.” “The question is,” said Alice, “whether you can make words mean so many different things.” “The question is,” said Humpty Dumpty, “which is to be master—that’s all.” Through the Looking-Glass – Ronald Sole – 2018-05-24T14:53:14.073
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@Tᴚoɯɐuo - For what it's worth, OneLook finds sixteen dictionaries that list highest, but only one that lists toppest.
– J.R. – 2018-05-24T16:25:37.0171
Shouldn't "top" be sufficient in most cases? "Top" already means "the highest point, level, or part of something", so there's little need to make it "even more superlative".
– el.pescado – 2018-05-24T16:45:36.3501
@FumbleFingers: It is a word that's been in use for quite a long time. https://www.google.com/search?q=toppest&num=20&newwindow=1&source=lnms&tbm=bks&sa=X&ved=0ahUKEwii_vL_7p7bAhXIqlkKHZj6AOwQ_AUIECgB&biw=1376&bih=1043.
– Tᴚoɯɐuo – 2018-05-24T17:27:01.9102@Tᴚoɯɐuo: Your "word" is my (facetious) "nonce coinage" (it's not in the full OED, for example). But is it useful for learners to know that some (tiny minority) of native speakers accept it as a word? – FumbleFingers Reinstate Monica – 2018-05-24T17:51:15.017
6@FumbleFingers Exactly. Toppest is not for learners. Using toppest will make a learner sound like they have failed to learn. Toppest for Jonathan Swift? Maybe. But not learners. – EllieK – 2018-05-24T18:44:49.493
1I will note that, although Thesaurus.com has synonyms for "Toppest" as noted by the OP, Dictionary.com (which seems to be part of the same site) does not consider it a word. – Kamil Drakari – 2018-05-24T18:54:48.517
And yet "bling-bling" is. Too many non-sensical words are included these days. – CrossRoads – 2018-05-25T15:47:54.893