I think Stumbleupon is by far the best. I've never used redgage, and I only had a fleeting experience on Reddit. I sincerely tried to engage on Reddit, but the layout of the website isn't really to my taste, and I can't navigate through it effectively.
Stumbleupon on the other hand is wonderful. I can tailor what I want to stumble upon, and while the interaction with other people leaves something to be desired, the layout of the website makes it easy to use. Plus, I find it's a much better source of inspiration than Reddit for example.