I don't think anything can make you feel as good, as appreciated, as worthwhile as doing something you love that is helping other people.
I think my 2 favourite volunteering experiences are
1) Reach Out...of course because it has helped me meet so many awesome people!!!
2) Life's Littles Treasures - this is a charity which helps mothers and families of premature infants. What i've gotten the most out of helping LLT has been that it has showed me how fragile life is and has made me appreciate what i have a lot more.