Peacekeeper shreds everything on hardcore, even at ridiculous ranges. Just be prepared for tons of idiots to toddler rush you for using a DLC weapon. If I'm not using the Peacekeeper, I tend to stick with the MSMC. Its iron sights and stability just work well for me.
Otherwise it's hardcore, so all the weapons are relatively effective. It's just a matter of what your playstyle is. The M27 is probably one of the easiest assault rifles to use for long range, and just in general really. The AN-94 is especially deadly if you have decent aim (especially on the first shot), but if your aim isn't very strong, be prepared to have a hard time hitting anything up close with it. The Type 25 is basically the opposite of the AN-94 - it'll annihilate anything at close to mid-range in hardcore because of its rate of fire, but you'll likely have a harder time hitting longer range targets with it.