Here's the best way to approach this...
First, if there is anything that really interests you or you are passionate about, I would do that first. It will definitely show (believe me, people can tell), and it won't really seem like a commitment when you are actually enjoying what you're doing... Generally these tend to be non-clinical volunteer opportunities, and that's fine. I would probably do something like ER volunteering (awesome flexibility) coupled with something you really enjoy. Do these for a prolonged period of time. This will be better than just adding more and more different activities/groups, which will start to make you look fake and also cut into your precious study and free time.
If you are really strapped for time or just don't enjoy volunteering (no one should judge you for that), then I'd do clinical volunteering coupled with non-clinical volunteering that gives you the following:
1.) Close to home
2.) Convenient work days and hours
3.) Preferably downtime to study during shifts
Ultimately, your MCAT and GPA will be most important. Therefore, regardless of how you feel about volunteering, the "best" volunteering will be the one that allows you to have a significant time commitment along with the best possible stats. Good luck!