How easy the language is to learn isn't that relevant. You should be thinking about the language that will help support your research -- particularly if you plan on going into a PhD program.

In my case it was Japanese (which I found impossible) and German (which was no problem). I use both languages to read sources for my dissertation. Even an "easy" language is very difficult to learn well. It's best not to waste time building unnecessary skills to save a little time in the short run.