The Resurrection of Christ, as is said in 1 Cor 15:14: “And if Christ be not risen again, then is our preaching vain, and your faith is also vain.”
Additionally, essentially all Catholic teaching can be derived from the Resurrection. There is no better way to explain our liturgical calendar, there is no better way to explain the significance of His Passion (and from there, the Mass), the Incarnation, etc etc.
I do have some reservations against making lists of the most fundamental truths though - not that they don’t exist, but some people use them to excuse their heresies through claiming that “we agree on the fundamentals”. Problem is we don’t - the Mass, for example, is unique to apostolic Christianity, and I’d say it’s a
very fundamental part of the Christian Faith. And **Christianity can’t be reduced to a lowest common denominator **anyways.
However, in the situation you describe, my reservations fall - for someone who is not acquainted by Christianity at all, we have to start somewhere. I’d say the best starting point is the Resurrection, others might say the Incarnation (kind of makes the chronology easier, I guess).
Edit: And before I was done writing, we have (including mine) four different answers from four different people
. That is the beauty of Catholicism in a nutshell.