They are all good IMO I get traffic from several social networking sites.. Here is a list of social bookmarking sites
Depends on what you mean by "best". Alexa wise? Brand wise? Digg.com seems to be the "sexiest" social bookmarking site right now, as far as people talking about it (people I know, anyway). I've only really used that and Stumbleupon, but I preferred digg.
Most my traffic comes from google. We have tons and tons of pages bookmarked in all kinds of social networks and all those back links help with the SERP's. As far as fast easy traffic, stumble a good page and get 10 of your friends to stumble it as well and you'll get 200 visits if the page has some good pictures on it. cheers
I personally like Stumbleupon because it continually brings in traffic. Second place would have to be Digg, than Reddit.