Haskell:

    case x of _ -> True 

is a valid expression of type boolean for all values of `x`.

    if b then True else False

is a valid expression if `b` is a boolean.

These can be nested recursively infinitely, but that's not very interesting, so let's move on.

    let x = z in y

this is another valid expression for any y and z values.

    do e

this is a valid expression if e is a type that implements the Monad typeclass.

Basically every other keyword is for type declarations or module importing, and those both aren't expressions, and seem much harder.

So, my final expression is:

    let x = x in do case if let x = x in True then False else False of _ -> return False

where `in do case if let` is my longest expression.