I have a Bachelor's degree in physics and I enjoy advanced calculus but my real interest goes to logarithmic/ polylogarithmic integrals and harmonic series.
I wrote a book about the harmonic series (An Introduction To The Harmonic Series And Logarithmic Integrals) and its available on Amazon.
Here is my best of on MSE:
$\bullet$ An advanced integral.
$\bullet$ Two powerful alternating Euler sums.
$\bullet$ Heavy Euler sum.
$\bullet$ Binomial sum.
$\bullet$ Cornel's integral.
$\bullet$ Three birds with one stone.