If you're prepared to roll up your sleves the ruck followed by the centre of the field are the easiest area to defend. Plenty of numbers about & always someone nearby to make up any misses. The further toward the sideline you get the harder the reads are!
I suppose hardest could be determined a few diffent ways. The workload, the unfit or lazy might find it tough going in the mid field. Or the decisions the outside backs or worse, Fullbacks are fronted with might be perceived as harder. I'm sticking with the latter. Anyone regardless of frame can learn to drop a bloke. It's the who & when decisions that make it hard for mine.