May Boolean circuits be exponentially more concise than Boolean formulae?
The circuit complexity of any boolean function of $n$ variables is at most $(1+o(1))2^n/n$, so a separation of between circuit and formula complexity of $\Omega(2^n)$ is not possible. This upper bound was established by Lupanov, and his method is known as the Lupanov representation of boolean functions. It also gives an upper bound of $(1+o(1))2^n/\log n$ for the formula complexity of any $n$-variable boolean function. See for example these lecture notes for a detailed presentation. Of course this does not rule out a separation of $2^{\Omega(n)}$. 

To the best of my knowledge the biggest separation known currently is $n^{3-o(1)}$ (over the de&nbsp;Morgan basis). It was proved by Håstad for a function with linear circuit complexity. This was an improvement on the method of Andreev, who showed a formula size lower bound of $\Omega(n^{2.5})$ for the same function using the method of random restrictions. You can check these notes for Andreev's proof. Håstad's bound is essentially tight for Andreev's method.

The class of functions computable by polynomial size formulas coincides with (nonuniform) NC1 and the class of functions computable by polysize circuits is P/poly. Separating these two classes is a well-known open problem in complexity theory, and is almost certainly well out of reach.