Flesh Crawlers Are probably your best best, I did them with all of my accounts that are 40def or less. Achieved 85str from 40 x2 there so pretty decent.
I think flesh crawlers are pretty much overrated, they are your best bet though if you dont want to constantly click since they are aggresive to any player for about 10mins regardless of your combat level. But with 40 defence their attacks are still pretty accurate. I would try zombies either in edgeville dungeon or at shadow graveyard since you can use clan wars to heal yourself and prayer. Or if you are looking for money try hill giants, they hardly hit on full rune.
So, if you want to train strength afk, pick flesh crawlers.
If you are looking for exp, pick zombies.
If you are looking for money, pick hill giants.