Hi Abolfazi,

keep cool! I am native speaker, but I learned from Miriam's answer. German grammar is sometimes a science for experts.

Ok, this is really tricky for learners and native German speakers alike. First of all: normally when adjectives are nominalised (e.g. die Ordentlichste), then they are capitalised. But ... there's an attributive elliptical use of adjectives: "If, after an adjective, a noun has been omitted that otherwise occurs in the text and can therefore be added just like that, one writes in lower case (= attributive-elliptical use)" (translation of Duden, rule D73, <a href="https://www.duden.de/sprachwissen/rechtschreibregeln/Gro%C3%9F-%20und%20Kleinschreibung" rel="noopener noreferrer" target="_blank">https://www.duden.de/sprachwissen/rechtschreibregeln/Gro%C3%9F-%20und%20Kleinschreibung</a>;). So, the omitted word here is "Mitbewohnerin". The complete sentence would be: "Sabine ist meine Mitbewohnerin und man kann sich keine bessere Mitbewohnerin wünschen". Thus, "bessere" is correctly written in lower case.

One could argue that in the following sentence, "Mitbewohnerin" was also omitted and one should write "die ordentlichste" but we can't be sure that it's referring to "Mitbewohnerin", it could also be referring to "Frau" or a similar noun. So, as we don't know for sure, that it refers to "Mitbewohnerin" and there's no other possibly omitted noun to be found, "die Ordentlichste" stands on its own as nominalised and capitalised adjective.

I hope this was helpful to clear things up a bit.