strongest 80/20 fastener

GMKlenklen,

If you are asking about the slotted 80/20, then you should use different connectors and biscuts depending on what forces the joint will encounter. With the slotted 80/20 that my former team used, we found that the connectors that can be purchased with the stock are more than ample in securing the 80/20.
Now if your question was about welds, possibly this site, and this site could be helpful, but I've little experience with welded frames/parts.
If you'd like more specific advice, I think I could be more effective if you described what type of joint you are trying to make and what, if any, unusual forces you expect to act on the joint.
Hope this helps,

Mike