Preferably pre-dental club if your school has one because they have events with dental school recruiters and other speakers like dentists, dental students, tips from pre-health committee, fun dental related events, etc.
To answer your second question, I'm not sure if dental schools are looking for specific types of clubs/organizations as long as you're proving your passion toward dentistry by shadowing, participating in pre-dental club's events (if applicable), and your willingness to give back by volunteering (there are many organizations on campus that are established mainly for that purpose so you can always join some. Oh and holding some officer positions will look really nice on the resume too). Good luck! 🙂