Search for the subject …
Ignoring the relative clauses for the moment, you can (somewhat) say
Du bist das Schönste.
Das Schönste bist du
Das Schönste ist: Du
Case one and two are basically the same sentence but case two makes use of the flexible placement of SPO in German. And all three sentences have valid grammar (although I have placed the colon in example three because my mind wants to say bist (*). The colon eases that desire a bit.)
In (1) and (2), the subject is du, thus the predicate is bist.
In (3), the subject is das Schönste (note that Duden says Adjektive in substantivischem Gebrauch werden im Allgemeinen groß geschrieben)
And a native German writer would probably evade the bist/ist dilemma by using a construct with man — like
Das Schönste, was man sein kann, ist: Man selbst.
which would (in this case) not even be an evasion, but rather closer to what the original sentence wanted to express — Assumed it was not used to address someone directly, but rather anyone.
(*) Note that if you say … something is something … it is perfectly valid to exchange the somethings (i.e. subject and object) in any language … But example (3) just sounds clumsy in native ears — Even if it is perfect German.