The most selfish thing? Not being selfish
the most selfish thing you could do is to be selfless
I intentionally changed selfless to not be selfish because I think it dials up the contrast, and better fits the example I'm going to narrate below. Also, as a non-native English speaker, I am ok with the slight difference there might be between the two terms.
Most of the examples around being selfless involve helping others, giving away money, etc. Those make a lot of sense of course but are also relatively difficult to practice often enough (except maybe helping others, which can be tricky as it might trigger the hero/saviour complex) and while I was discussing this with my wife I failed to come up with her rebuttal that this sounds a lot like the warm, fuzzy stuff you find yourself nodding to when you read it on motivational posters while knowing you will never practice it.
Over the corse of that same evening I did however find a way to practice it, and perhaps prove the point of the saying too.
That evening, after a filling dinner, I wanted to take the car and get ice cream too. There's a really good shop close-by and I wanted to indulge myself. Being the much more reasonable person that I am, she said I should not go. When confronted with my stubborness she made it very clear she would not be happy about it.
At which point I concluded I was better off not going, in the long term.
And together with that decision I laid out my reasoning: getting ice cream now was the selfish thing to do in the moment. However, not getting ice cream now was the most selfish thing to do in the long term.
Enough of these short-term rewarding choices would eventually end up putting strain on our relationship, so it is in my best long-term interest (being selfish) not to be selfish (in the now).
The astute reader will notice that this applies to a lot of interaction we have in a professional setting too.