There are loads of ways of designing tyres either by using computers or making pro riders go round and round in circles across different terrain (or both), but we really like Specialized's pragmatic approach to their new cross-country tyres. The Chunder XC basically looks like a skinny, worn out version of their Chunder DH tyre. It has the same tread pattern, but the paired centre knobs are really low, so they roll super quick. It sits on a supple, lightweight 1.9in carcass too, so acceleration is easy and instant. The side knobs are full size though, so if you get them onto their edges there's plenty of bite for creating your own berms.
The Chunder XC is designed as a summer tyre but we've used it on a rear wheel all winter (with a Storm tyre up front) anyway because it's fast and great fun. It fishtails under power in mud and it can spin on climbs but it's easy to predict when it'll slide in corners and how far it'll go. Dual medium/ soft compound rubber trades good grip on rocks for fairly rapid wear, but we're really impressed with the pinch puncture resistance after crushing two wheels without popping the tyres.
The price is good too, so all we're left wanting is a larger 2.2/2.3in size for bigger bikes.