Esperanto! Yes, there are better conlangs, yes, it’s eurocentric, and yes, there are ways to improve it or even come up with something better. But it has a cool history, it’s tied to socialist movements and anarchist movements, it is fairly easy to learn (especially for speakers of European languages), it’s grammar is super simple, it uses a system of root words and affixes that make me think of Legos, and it has real, native speakers already, meaning it is a living language that has changed over time, and is fully capable of being used exclusively to communicate efficiently.

Plus, the fascists fucking hate it