Imo, I'd say it depends more on the type of RS player you are, it's hard to say which would be better and why because it's kind of like a mini-combar triangle within the one of Magic>Ranged>Melee>Magic. I'd honestly say Defence would be one of the most valued out of the three even thought I hate it, mainly due to the ability to 'tank', also you last longer on higher leveled oppenants and therefore save more food than lower levels would, but then again, Defence would be useless without a decent Attack and Strength level to match it. After all, what's the point of never getting hit if you cant even finish off an enamy?

 

In other words, I say they are never useful without the other two.

My personal favorite would be attack.

I'd rather have consistent, medium hits than rare high hits.

 

Second I would have to say defense.

Especially since I've started doing slayer, it's really helpful to have high enough defense so I can do almost all of my tasks in one trip.

 

All of them are important, though.

I prefer to keep them even & balanced for the most part ;]