Cold, generally for me, although I might have hot on Weetabix. But as I don't have a microwave, and washing a pan is work, I'd use hot water from the kettle, and a dash of milk on that - I'm actually not that keen on the taste of heated milk. Put that on All brm and you get a very healthful brown slop that wctually tastes quite nice.
Porridge is made with 50/50 milk/water, and that is worth the washing up.