Heater hose sounds like it to me but I don't know as much as others on here as it is.
When my system blew it was the radiator and associated hoses. I floored it from a green light to beat traffic from the light since my lane was ending, and it was a very hot day so that had an effect too probably. Pretty embarrassing when you fly by everyone only to be passed by them 30 seconds later with steam everywhere.
I also saw an e38 yesterday parked on the curb with a large trail of coolant behind it.
Such a well designed system.