I’m a native speaker, but I’ll venture this.
Joachim Pense correctly said Classical Greek—and he also said that if you don’t already know Classical Greek, it is something of a detour.
Knowing any language which has taken a lot of vocabulary from Classical Greek—meaning all Western European languages other than Icelandic—will help the vocabulary—but less than you might think. Knowing Italian or Turkish used to help you more with the vocabulary than it does now, because the revived Classical Greek words displaced them.
For the syntax, Bulgarian and Macedonian would help. Albanian would help for the syntax and the inflection overload (at least to get you over the shock of how much there is).
But Greek is its own language branch, so no language is going to give you the kind of leg-up you would get in other, larger language branches.