Ideally well seasoned wood that will not give off too much smoke to choke your guests or neighbors.
If you are intending to cook food on the embers, then obviously you don't want to poison anyone with wood that's been painted or treated with chemicals in the past.
I like to start off with kindling, to get it going then put on some hefty slow burning logs later on and maybe some charcoal once you have a decent bed of embers.