[tome]

I did extensive research on conlangs about a decade ago. My conclusions were that the conlangs worth actually trying to learn were

* Esperanto, because a significant number of people already speak it

* Ido, because it's a significantly improved version of Esperanto

* Novial, because it is significantly more rationally structured than Ido

* Interlingua, because it's a significantly more rationally structured version of languages around 700 million people already speak