Carbon crushed or in crystal/cube forms?

As with any chemical, the most efficient form tends to be the form that has the most surface volume exposed to the water. In this case, the answer would be the form where it's smashed the most down inside of a mesh-like bag that can hold it in place but allow for water to pass through it.

I normally tend to place carbon (when I use it), somewhere that can be serviced rather easily due to its small lifespan within the water - whether this is specifically placing it in a HOB filter, or when in a canister filter, in the top tray to make it easy for quick removal when the time comes.