Mountain biking is not what people think it is. Looking at it from the outside, people would think you ride a beefed up bike with knobby tires and suspension because you are riding gnarly trails full of rocks and roots. The reality though, is that in order to go fast, in order to be considered to be a "good" mountain biker, you avoid as much as you can. Picking the smoothest and thus fastest line is the name of the game.
I did an experiment once where I stripped my mountain bike and converted it to single speed. I felt like my handling wasn't the greatest so I took away the distractions of shifting and focused on picking good lines, on going fast. It worked quite well and I even managed to win a race against geared riders in the process! I was loving the simplicity. It never broke, never needed maintenance. I could just get on and go fast.
I eventually ended up switching back to a geared bike and when I did, I opted to try SRAM XX1. Some folks compare it to riding single speed, and some to riding 2x10; Its like riding both. XX1 is by far the fastest group out there. Why? Because it works when going through hell.
Putting XX1 to the test is easy if you get a nice rainy forecast for a 100 mile mountain bike race. I had the opportunity at the 2013 Cohutta 100. I raced in the mud and rain all day. My entire body broke down at some point, but my drive-train was perfect, all day. Never missed a shift. Phenomenal.
XX1 is simply faster than the rest. You will be slower if your shifting slows down on a rainy day. You will be slower if you are shifting front rings and back rings while thinking about which is best. You will be (marginally) slower if your group weighs a third of a pound more. You will be slower if you drop your chain. You will be slower if you damage anything from your chain slapping and bouncing around. You will be slower if you suck your chain. Simplicity for a group that is fighting against the elements (literally) is king. I don't think I've ever met someone who rides single speed who didn't do it because "it is simple and just works." SRAM XX1 provides you the range you need to be competitive with the geared riders, without the hassle of breaking parts or mis-shifts!
I ride SRAM XX1 because of its reliability, and simplicity. It just works, and really well at that.
You can pick up a SRAM XX1 group here.