What's your favourite type of differential equation

[J77]

Just a small list, mark up any others I have neglected...

edit: already see, I should've put in PDEs with delay...

 

[mathwonk]

my fave is the several variable complex heat equation, to which riemann's theta function is a fundamental solution.

 

[Clausius2]

I've started to enjoy those equations with a small $$\epsilon<<1$$ just in front of the highest order term...