Well, I know the best armour currently is called Tourva (I think) But for slay, it really depends on the task. But obviously slayer helm, dboots, fury, berserker ring, barrows gloves, firecape. The shield, and armour are dependant on your task.
The best armour for slaying would have to be bandos/black dhide depending on the task. Torva is the best but it degrades so you'll be loosing a lot of monye using it.
for a slayer task.. most of them anyway
Slayer helm
Amulet of Fury
Firecape ( substituted for ardy cape 4 or 3 if you want)
DFS or dragon defender
Barrows gloves
D boots
Torso ( cheaper version of bandos chestplate, but less defence)
Bandos Tassets
Whip, sara sword, rapier, cls, dosnt really matter.. hope it helped
If you're going to slay then I recommend you getting; Bandos/Fury/Firecape/Dragon Boots/Slayer helmet/Barrows gloves/Berserker Ring and a Chaotic Rapier, I hope that helped ^_^
If you are after armour that is good and not too expensive, I would suggest Dharoks, as that also has good strength bonus. Also if your are going to use the whole set, Guthans is very good because of the special healing it can perform.