Well, certainly the first to be widely implemented. After the original Fortran for the 704, there were compilers for a bunch of other machines, not always using the "Fortran" name. And, per this article, the first to have a meaningful optimizer. (Terms such as "basic block" first occured in 1957 technical papers on its internals.)
However, there were earlier languages which were at least high-level enough to support algebraic notation. Perhaps the first the compiler written by Laning and Zierler for the Whirlwind at MIT: https://en.wikipedia.org/wiki/Laning_and_Zierler_system (which was thought for a while to have been an inspiration for Fortran -- but the head of the Fortran project, John Backus, later found notes from before Laning and Zierler published).
