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.30-30 if we're talking about "cowboy" lever actions. If I have my choice of lever actions, then I'd pick a Browning BLR in .308 Winchester. While I understand the attraction of having your rifle/carbine chambered in the same caliber as your pistol, I think it is too limiting. The whole point of having a rifle is to gain power, range, and accuracy over a handgun. When you shoot a lever rifle chambered in a pistol caliber, you gain an incremental amount of velocity....but it's not like twice as fast or three times as fast....so the power gain is not dramatic, and neither is the increased range. The main advantage is a longer sight radius.

OTH, the .30-30 cartridge from a carbine length lever rifle roughly duplicates the ballistics of the 7.62x39 cartridge—which is certainly potent enough to be an effective "something arises" caliber. But a BLR in .308 gives additional power yet, and it uses a removable box magazine, so reloads can be pretty fast. That woud be a significant advantage in a "something arises" rifle (which is one reason why ARs and AKs are such good choices for this role).