Spanish
These types of rankings nearly always rank Spanish high, and with good reason. With 477 million native speakers and more than 550 million total speakers, it is the official language of more than 20 countries such as Costa Rica, Argentina, Chile, Cuba, and Mexico. In the United States, where 13% of the population speaks Spanish at home, it is also an unofficial second language.
Even if you aren't taking in-person classes, getting practice material shouldn't be difficult because there are a lot of other Spanish online guides. You may practice your comprehension by watching one of the many Spanish-language Netflix series, including the popular favorite Money Heist. As your proficiency increases, consider watching without the English subtitles.