That's a bit of a strong claim to make, considering that there are thousands of languages with more speakers than Esperanto, most of which you probably never heard of. My guess is that some contact language has similar difficulty, while having more speakers.
Pichinglis [1] seems like a good candidate: 70,000 to 100,000 speakers in Equatorial Guinea; formed as a creole from English and various languages of Western Africa, so you should get a high number of cognates similar to Esperanto and a simplified grammar. It might even help you learn other languages of the region faster.
About as good a choice as learning Esperanto is for someone who doesn't plan to go to an international Esperanto conference, i.e. not very.
It's probably better to learn a language when you think it'll help you with something you're already planning to do, rather than pick a language and then try to find opportunities to use it. (Also applies to programming languages.)
Pichinglis [1] seems like a good candidate: 70,000 to 100,000 speakers in Equatorial Guinea; formed as a creole from English and various languages of Western Africa, so you should get a high number of cognates similar to Esperanto and a simplified grammar. It might even help you learn other languages of the region faster.
[1] https://en.wikipedia.org/wiki/Pichinglis