Which of the following figures has the greatest number of lines of symmetry?
$\textbf{(A)}\ \text{equilateral triangle}$
$\textbf{(B)}\ \text{non-square rhombus}$
$\textbf{(C)}\ \text{non-square rectangle}$
$\textbf{(D)}\ \text{isosceles trapezoid}$
$\textbf{(E)}\ \text{square}$
An equilateral triangle has $3$ lines of symmetry.
A non-square rhombus has $2$ lines of symmetry.
A non-square rectangle has $2$ lines of symmetry.
An isosceles trapezoid has $1$ line of symmetry.
A square has $4$ lines of symmetry.

Therefore, the answer is $\boxed{ \textbf{(E)}\ \text{square} }$.