I have worked on all three brands (a SLR was a ION Ultra Lite, BCM MCMR and Geissele MK14) and like the Geissele MK14 the best, followed closely by SLR and view the BCM as a step below.
The SLR and the Geissele both have integrated QD sockets while the BCM comes with a QD sling mount. The BCM get some points for the included barrel nut wrench but the wrenches for the SLR and the Geissele are not that much more of an added expense.
All three are great after they've been installed. The BCM does have the cleanest transition from rail to receiver. For me it boils down to the barrel nut and the anti-rotation design. The BCM barrel nut is easy to mar when tightening and/or loosening it. The SLR and the Geissele both have superior barrel nut designs (in terms of length, cross bolt clamps and wrench interface) but I believe the Geissele is the better of the two. I also prefer the anti-rotation set screws on the Geissele over the anti-rotation inserts on the BCM and the SLR.