Fire/Ice/Thunderpunch is a good start. I'd settle for those three so people can choose what they want, and I have seen Dusknoir use at least one of the punches on pretty much every set, so this gives the trader the choice of what to use.
Dusclops/Dusknoir can relearn all the punches with the heartscale tutor in D/P so it would be a waste of BP.
Edit: According to Serebii's page these are the tutor moves dusclops can learn.
From FR/LG/E:
- Mega Punch: useless
- Mega Kick: useless
- Body Slam: paralyze is good but there are better moves it learns naturally.
- Double Edge: : useless because Dusknoir doesn't have good HP and the recoil will kill it.
- Counter: useless because lack of HP means counter hits poorly.
- Seismic toss: I'm pretty sure it learns Night Shade and even then nobody uses it.
- Mimic: gimmicky.
- Metronome: gimmicky.
- Dream Eater: gimmicky.
- Rock Slide: I actually think this one could be good but thunderpunch is generally a better idea.
- Mimic: gimmicky.
- Substitute: is a TM in D/P.
Move tutors exclusive to Emerald:
- Dynamicpunch: without no guard I think is too much of a gamble.
- Psych up: gimmicky.
- Snore: gimmicky.
- Icy Wind: gimmicky.
- Endure: With no flair or reversal I don't see the point.
- Mud Slap: gimmicky.
- Ice Punch: learns it with heartscale in D/P.
- Swagger: gimmicky.
- Sleep Talk: is a TM in D/P.
- Thunderpunch: learns it with heartscale in D/P.
- Fire Punch: learns it with heartscale in D/P.
Hope that helps.