The first 2 years of undergrad at my uni are calculus, discrete maf, linear algebra, and statistics. That’ll grab all of your math minors, phys, engi students probably. Beyond that, analysis or or maybe graph theory and topology would be popular? As far as I am aware, real analysis 1 is usually where people drop out of math majors lol

Edit: number theory for sure lol, but I believe there are already a few good number theory proof channels