I am very fond of TikZ. In particular, I really like doing 3D illustrations (even though TikZ is a 2D software). Some of my favorite work so far includes

1. Parameterized the drawing of planes (based on normal equation) which are clipped by a rectangular prism.
2. Parametric surfaces are fun to make, though I never was able to sort surface triangles in 3D - at least not yet. The correctly drawn surfaces were just me getting lucky with centroid sorting.
3. Charge induced vector fields are really fun to make, and are really simple too.
4. I really like making stereographic projections; I think they look really cool.

A few more interesting animations: