Effective altruists want to find the one cause that is more effective than all other causes. As a result, EAs often debate amongst themselves what the most important cause in the world really is.
The reality is that there is no one true cause.
For one thing, different people are good at different things. I could start training right now to become the world's best brain surgeon or rocket scientist so that I could solve some highly important issue in those fields, but it would probably make more sense to let the current brain surgeons and rocket scientists handle those issues, while I work on the important issues closer to my current background. You know, the type of stuff that brain surgeons and rocket scientists wouldn't be able or willing to pay much attention to because they're too busy working on their own problems. If I felt like I had the capacity and motivation to train myself as an expert rocket scientist so that I could find the rocket science Holy Grail and benefit humanity, that could be a good idea. But there are huge issues in a diversity of fields and there's likely to be a Holy Grail hiding closer to my present destination.
Secondly, because there are so many different issues out there, we should want people to be working on all of them. If we collectively identified aging as the world's most cost-effective cause and everybody dedicated themselves to that, maybe we'd manage to cure death... only to get wiped out by some existential risk that nobody was paying attention to. So the goodness of a cause depends on how much attention is paid to it relative to the attention paid to other important causes. As more and more people start focusing on the "top causes," that might make formerly lesser causes surpass them in importance due to their relative neglect. The importance of a cause is constantly fluctuating, a little like the stock market. Whenever someone decides to focus on something, that changes the worth of focusing on that issue.
Effective altruists should keep their eyes and ears open for new high impact causes. They should also pay attention to what other people are paying attention to. In some cases, it might be wise to join a highly rated pre-existing EA movement. In other cases, it might be better to join a pre-existing movement that is usually deemed by EAs to be of less importance. In other cases, it might be best to start an entirely new project that has yet to be seriously explored or dealt with.