Cheapo leather chair from a highstreet catalogue shop. Nothing special and my partner's fabric office chair from an office supplies store is actually more comfortable.
At my dad's house I have a cheapo fabric office chair which has had so much wear that most of the 'foam' padding stuff has been pushed to the back of the seat, so it's not actually flat any more! The bit I actually sit on is really thin so you can feel the plastic underneath. Still, it must be around 8(?) years old now so not bad going for £30.