I just posted the link to the moon guide on another answer, but here's the link.
If you go to the last graph, it shows after an update the investment that will make the most (long-term) is the lasers. Second is Helium-3 farm, so I would choose either of those if you want long-term. The graph is hard to read, since it shows the old and new lines, but another option is the oxygen bar. Oxygen bar is short-term, but it's one of the ones with a large milestone (to hit 2880) and gives a big boost when you finally hit it. Otherwise, just pick your biggest money makers and you'll get a slight boost.
It depends on your stage of the game and level of advancement of specific investment. If you're stuck and planning to hold more of Mega Ticket, you can use it on the current most profitable investment, but it's better to think long-term and place it on the more expensive investment such as laser.