The fundamental research question of effective altruism is, “How could I act so as to maximize the world’s goodness?” Another way to formulate the same idea: “How could I act so as to maximize the amount of good I can do over the course of my lifetime?” This question could also be translated as, “What is the most important cause I could possibly be working on?”
I think something important gets lost in translation. The third question is confused because it assumes that, if we could only identify the most important cause in the world, then everyone could maximize utility by simply working on that most important thing. But the optimal world is going to have an optimal equilibrium of several factors. We don’t want maximum resources invested in the World’s Best Cause. We want maximum resources spent on realizing the Best World Possible. As a mass strategy, it will be best for everyone to stick to what he or she already does best, adjusting a little toward effective altruist values. If every effective altruist with a business, arts, and social work background jumped ship to do artificial general intelligence research, there would be nobody sane left to run the other industries. If most people stick to what they do best, the mass allocation of resources will be more cost-effective.
The second question confuses “the world” with “my lifetime,” an easy mistake to make. But effective altruism isn’t about getting into the Guinness Book of World Records for Most Lives Saved Ever, it’s about maximizing your impact. In some cases, personally doing less good will allow for more total good to get done.
Even the first question could be vulnerable to criticism, depending on your views.
Firstly, Negative Utilitarians might disagree with the use of "maximizing the world's goodness." This might be interpreted as focusing too much on the maximization of wellbeing, compared to the minimization of suffering. I think this constitutes a disagreement with how we should define "the world's goodness" more than it does a disagreement with the idea that (correctly defined) maximized world goodness should be our target.
Secondly, as I argued in Anchoring to Other Perspectives, aiming for the altruistic stars might, on average, lead to worse consequences than would more humble altruistic efforts. This is again more of a disagreement with the strategies that should be used for world optimization than it is a disagreement with the idea of making the world as good as we possibly can.