The antonyms of the phrase "most out of sight" are "most visible" or "most obvious". These terms signify something that is easy to see or notice. They also denote presence and prominence, as opposed to "most out of sight", which implies concealment or inaccessibility.
For instance, in a game of hide-and-seek, the player who hides in a highly visible spot can be said to be "most visible" or "most obvious". On the other hand, the player who hides in a dark corner or behind a large object can be considered "most out of sight".
In summary, while "most out of sight" pertains to something hidden or obscure, "most visible" and "most obvious" are descriptive of objects or individuals that are highly conspicuous and easy to detect.