$\text{ALL}$ is literally the class of ALL languages.Are there $\text{ALL}$complete problems?  That is, are there problems for which a solution would allow one to solve any problem whatsoever?  Such problems could reasonably be consideredthe hardest problems, bar none.One such problem seems to be: given a problem and an input size, output it's truth table.
Problem complete for the class of ALL languages
undecidability
You haven't specified your notion of reduction, so I will assume that you choose some countable class of functions $\cal F$ which can be used for reductions (any subset of the computable functions would work here). Let $\cal L$ be any class of languages over some fixed alphabet $\Sigma$, say $\Sigma = \{0,1\}$. A language $K$ is hard for $\cal L$ (with respect to $\cal F$) if for every $L \in \cal L$ there exists $f \in \cal F$ such that $x \in L$ iff $f(x) \in K$. If also $K \in \cal L$ then we say that $K$ is complete for $\cal L$.I will now show that no language is hard for $\mathsf{ALL}$. Suppose that $K$ were $\mathsf{ALL}$-hard. Let $f_x : x \in \Sigma^*$ be an enumeration of the functions in $\cal F$ (such an enumeration exists since both $\Sigma^*$ and $\cal F$ are countable). Define a language $L$ by$$L = \{x : f_x(x) \notin K \}.$$Since $L \in \mathsf{ALL}$, there exists a function $f \in \cal F$ such that for all $x \in \Sigma^*$, $x \in L$ iff $f(x) \in K$. Since $f \in \cal F$, there exists $x \in \Sigma^*$ such that $f = f_x$. For this particular $x$, $x \in L$ iff $f_x(x) \in K$. However, by definition $x \in L$ iff $f_x(x) \notin K$.