Why do you think English would be least compressible? Is that based on conjecture or have you investigated this? Why would artificial language be more compressible? That seems completely orthogonal to me (by definition, an artificial language can be designed with whatever properties you choose). Fortran may be more compressible due to its limited set of keywords, but it's my impression that Ithkuil is by design more information dense and thus harder to compress than English.
The most efficient language is the least compressible language only in a narrow and arbitrary sense of efficient. There are many considerations such as what is efficient for the speaker, the hearer, redundancy to noise, efficiency with respect to particular purposes, etc. We can assume that natural languages will generally make a good trade-off across these factors, and searching for the most efficient language in one particular narrow sense is not very useful. Moreover, compression of text focuses only on surface form, completely ignoring the dimension of meaning.
The most efficient language is the least compressible language only in a narrow and arbitrary sense of efficient. There are many considerations such as what is efficient for the speaker, the hearer, redundancy to noise, efficiency with respect to particular purposes, etc. We can assume that natural languages will generally make a good trade-off across these factors, and searching for the most efficient language in one particular narrow sense is not very useful. Moreover, compression of text focuses only on surface form, completely ignoring the dimension of meaning.