Which varieties make good root-stock, for grafting?

[tomatoguy]

I would be interested to know which plants, in your experience, have the most awesome root systems. I may do some grafting next year but I prefer to try something original. I'm sure the commercial root stock varieties are great but I also know there are some standard heirloom varieties with awesome root systems. I have seen monstrous roots on Granny Cantrell's German Red, Polish C and Roger's Best Black. Care to add any more to the list?

mater