The 300 WSM
for a couple of reason. First is you said it shoots the best of the 3. Next is, it delivers a larger bullet on target. If you are loading it to fit the mag box on a short action, then you are losing some potential. I would strongly recommend looking at shooting the 215 hybrids over RL17. That would make it a 1000 yd rifle. If the 215 doesn't work for you, go with the 210, also over RL17. I was able to push them comfortably to 2930 out of a 24" barrel. The 215's might even get a little more velocity because of their shorter bearing surface.
It's a toss up between the other 2. I'm not a belted case fan, but the 7 RM will have a little more capacity than the 7 WSM. Also recommend looking at the 168 and 180 VLD's. Retumbo or H1000 for the 7 RM, with Retumbo being my first choice and H1000 for the 7 WSM. I got best velocities with Retumbo in my 7 RM. I have heard RL33 also does well in them as well.
The heavier for cal, higher BC bullets will always be your friend. Trading velocity for mass and BC is a good thing.