Generally whoever you can get to.
Usually it's when you need to catch up to someone or they're trying to get away. Because of the way the on-hit works, even if you need the active to win the duel you want to save it for the very end of the duel since your autos deal %current hp while the active is %max. Also keep in mind if you suspect they will be using an ms boost soon so you can leech off of that, too.
Almost never rush it unless you're doing poorly and you plan on it being your only damage item. If you're doing well then Youmuu's is better and if you're doing poorly you generally want more durability from a sunfire or dead man's.
Frozen Mallet isn't too popular but definitely helps out within that role. IBG is good if you can trigger it a lot (i.e. nasus with his low cooldown AA reset) and/or if the enemy has enough AD that you want some armor.