That is incredibly relative... Test them out for yourself and see which one(s) work the best as your mileage will always vary compared to someone else.
I love the propeller or newsvine as the visitors are genuine if compared to digg, stumble and delicious.
I get traffic from Digg, Propeller, and Stumbleupon. I have gotten absolutely NOTHING from del.icio.us. The way I see it though, Digg and Propeller aren't really social bookmarking sites in that they allow you to actually SUBMIT your site, or new article to their directory. Stumble upon and del.icio.us are just simple bookmarking sites. I don't understand what all the fuss is over del.icio.us. If someone can give me some insight into how to get traffic from del.icio.us, I'd love to hear it.