Hello up in t'north! As you know I normally ride in the soft south but the hardtail's come with me to a fair few rocky places. And the answer is Rubber Queen aka Trail King 2.2 Protection.
My library of comparative tyre comparisons suggests that they're faster rolling than the On-One combo - in the dry they roll as quick as a Mud-X but have a ton more grip. In the mud they're draggier than the Mud-X but have comparable driving and braking grip and more cornering grip.
They're high volume for their width so great in the rocks. They are sticky, like 50a or stickier but roll like 60a or quicker. The tread is freakishly long-lasting which easily offsets the high price (cheaper from Germany) - I had them on for a year before I thought I'd try some mud tyres last month (Baron & Mud-X) and there is very little wear on them. Their only weak points are they need a reasonably wide rim because of their high volume, in properly loose conditions they could do with slightly taller edge knobs and I gather the non-Protection or UST ones have a flimsy carcass. They'll be going back on as soon as the weather turns!
I've had the old XR4s too and they were similarly voluminous but the carcass was less tough, the compound less sticky, the braking/driving grip worse, and they wore quicker and the edge knobs deformed under hard cornering so they drifted sooner. The Purgatory 2.3 isn't quite as voluminous, isn't as sticky but is less heavy going in the mud.
A slower rolling alternative with more edge grip but less volume or tread life is the Butcher Control 2.3 - better as a front than a rear. If you don't mind possibly watching sealant leak out for weeks then the Baron is a great front tyre for looser conditions with even stickier rubber. Lower volume and the carcass isn't as tough because they don't do a Protection or UST.
If you have the frame clearance and value grip highly but still want a tyre that doesn't drag horribly I think the RQ 2.2 BC UST / Protection is as close to the perfect hardtail rear tyre as it gets. And when it's more rock or dirt than mud or loose loan it's equally good on the front. I don't know if they still sell the BC UST version - I have a spare set for when these wear out because they're so good - it might have been replaced by the Protection version now they've made the name less amusing for 2014; anyway, the UST carcass has a really nice feel on the back of a hardtail, it still rolls well but it has this wonderful self-damping more like a thicker heavier tyre that makes the back end feel noticeably less fidgety when you're pinning it.
Sales pitch over!
P.S. Do I win a prize for the longest post about one tyre?