Toki pona. Despite only containing one-hundred and twenty words, it manages to maintain some active communities with speakers of itself. How does toki pona manage to keep the vocabulary in the language so low whilst being able to explain many things? Well, first of all, it generally relies on context quite a bit, with lots of words meaning many different things. For example, the word for the sound an animal makes comes down to a single word - "mu". It also, being an artlang, seems to be a lot more poetic in the way it is spoken, with words outside the vocabulary given being formed by combining the limited words to create concepts. For example, something like radio can be translated to ilo kalama (a tool of sound). This has led to not only many different interpretations of words being formed, but also has led to some interesting combinations of words in order to explain more complicated concepts that wouldn't even require thinking in a less-minimalistic language. I feel it's main strength is the freedom it offers to the person communicating in the language, and forces them to be more creative with usage of the language in order to get their point across, whilst also allowing the language to keep it's vocabulary count low.
Now why should you learn toki pona? After all, in comparison to conlangs like Esperanto, the number of toki pona's speakers is quite low, however, this is where the nature of the low vocabulary shines in learning toki pona. With toki pona, you can get a solid grasp of the language all the way from a few days to a fortnight. Not only that, but learning a conlang like toki pona allows you to see your language learning process when moving onto more difficult languages, without having to invest the years that might go into learning another language. Also, like previously mentioned, it does encourage you to think about the way you communicate no other language (that I know of) does. All in all, toki pona is not much of a time sink to learn (I managed to cover the grammar and vocabulary in two days, but even on the longer side of things it generally takes people up to as month, and has many potential benefits - so why not join the small but lovely communities around many areas?
Tl;dr: Learn toki pona, it's cool.