English, without a doubt.
English is not only the original and most widely used language online, and the language of two suporpowers (UK and USA) with wide network of allies and interests, but there are market reasons.
In English speaking countries, there is a big for-profit market for tool/courses/resources to learn other languages. Because it is so big (and has many paying customers), price can go down substantially. There are many massive online platforms like edX.org, coursera.org, udemy.com with huge number of free (or extremely inexpensive) courses. They sell their services to companies interested in their employee's education, and as a result can sell big volume of such courses, so individual price is very low. Also, because of such competition, many smaller companies publish free material as loss leader to get traffic and human interest to their websites, hoping to sell some additional products.
In example edX.org provides most courses for free, and pays for the servers by charging $50 for certified courses (where they provide certification that given person passed the course). But courses are free without the certification. Also, old courses are archived and you can read it all for free, even read the archived chats. The only thing missing in free course is: you cannot get a response from a real live teacher (only from your peer students).
This is economy 101: if products are sold in big volumes, per-piece price is close to marginal cost. And marginal cost of a digital file hosted on the internet is close to zero.
Also, both UK and USA spend non-for-profit and government resources to promote English and also created free resources to learn both English, and learn other languages for English speakers (and published them on the internet): Both "Voice of America" and BBC have big online presence (funded by governments with budgets bigger than most other countries can spend on such efforts).
FSI is required to sell educational materials for USA citizens at cost (because their creation was financed by taxpayer's money). They resolved it by just posting it all (instructions for more than 70 languages) on internet for free.
As a result, there is huge volume of free or very cheap language learning resources in English, and problem is not to find a resource, but decide which one from so many available is good enough to spend your time on.