I like sharp shoulders, very little case taper enough neck and no belt. The brass should come from Lapua, Norma if possible.
Size the capacity to speed/bullet weight needed. In fact I like a little extra capacity so I don't need to lean on it to hard to get where I want to be.
I like a case short enough that there will be a lot of COAL lee way and still fit in the Mag box.
Cartridge popularity means nothing to me. When I have something put together I will usually get enough brass to last quite a while.
Some of my favorties currently in the safe are 223AI, 250AI, 6.5 CM, 7mm-08AI, 7mm SAUM, 300WSM, 338RCM and 375 ruger.
