最&高

Read literally, the commentary text is just "saikou" (The best, the greatest, ultimate, etc.) with its kanji split by an ampersand.

Most likely it's a reference to Sai & Co, a single released by kyary pamyu pamyu to commemorate her 5th anniversary as an entertainer (see the lyrics in the wikipedia link, they're very applicable).

Similarly, this image honors Pekora and Moona finally having "their":https://www.youtube.com/watch?v=rIh3wjETcD4 "collab":https://www.youtube.com/watch?v=QivLpdnBgUs after so long, as well as Pekora hitting the 1 million subscriber mark (note the Youtube awards on the wall).