I'm about the least picky rider around, so take my opinion with a grain of salt...but I've been very happy with my Bontrager XR2's the last two seasons. Good balance between a fast roller & mega gripper. They are 29x2.35 & my rims are probably in the 21mm internal ballpark (Giant SXC2's from 2011...told you I wasn't picky).

My previous setup was Specialized Ground Control 29x2.3 up front & Fast Track 29x2.2 in back. I am a fan of the better rear end traction (that's what she said) of the XR2.