Here's a series I like

I start by using some of my building block formulas for rapid prototyping:

$eq1=atan($x/$y);
$eq2=cos($x*$y);
$eq3=sin($x*$y);
$eq4=sqrt($x*$y);
$eq5=cos($y*$i);
$eq6=cos($x*$i);

Then plugging a few of them into an equation like this:

$eq=round((log10($eq1))*(log10($eq4)*($eq5+$eq6))*$i);

I added some random color code to my algorithm. I think it does a good job of keeping things fresh: