generally the most expensive you can afford, unfortunately.
try and go for a fine grain to start, they are a bit more forgiving untill you master the angle at which to brush your knife down it, they come in all sorts of differnt grains, from super fine right up to harsh allmost wet stone grain.
they usually come in 2 shapes oval and round, the key to using a steel is little and often, 2 strokes either side of the blade, not too much presssure from your arm / wrist, and not extreme on the angle between knife and steel, do this as many times as the workload in front of you will allow.
so f