Conty XCs are good for efficiency, but they really aren't much fun.
Sauserwinds are also really good, though I thought I heard they weren't making them in 1.8 anymore? 2Bliss versions would be the best skinny out there I reckon.
There was some research done recently that points out that apart from dead-smooth terrain, larger diameter large volume tyres roll better (assuming the same carcass construction etc).
Utterly unscientifically, I'd go along with this – I've recently fitted those cheap Michelin XC Dry 2.15 from on one (£10 an end for folding and went up tubeless no problem) – it appears I'm rolling faster, noticeably so to other people. They grip pretty well at low pressure, but I think mud would be beyond them.