Agree with what most people have said on here.
In the B738W you really are splitting hairs to be too concerned at the level you hold at. There really isn't that much difference in your fuel flows between say FL300 - FL150. The best level will be the one (generally) that fits your profile when ATC clears you to exit the hold in my opinion. Obviously there will be other considerations such as traffic, ride, icing etc etc
But, that being said, if you were to hold for an extended period (more than say 30mins) and had a choice of level, FL250 would give you the lowest burn for a given weight. eg 2160 kg/hr @65T B738W