What is the best mountain bike drivetrain for the money?

It's one thing to offer the best mountain bike drivetrain; it's another to deliver the best value.
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Most of us are running a mountain bike drivetrain from a brand that starts with the letter S, but that still leaves a wide range of options when it comes to groupsets. Considering both the performance and the price, we want to know which of these modern drivetrains is your favorite. Then tell us why in the comments!

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What is the best MTB drivetrain for the money?

And if you prefer to run fewer than 12 gears on your mountain bike, we want to hear about that in the comments too.