Why do you want to revive them? If it is for you, then look for features you want and a size you need. If you want to refurbish them to give away to the needy or to sell at a church sale, then check for "common" sizes and newer parts such as indexed shifters. And if you want to revive vintage bikes, then go in the dump yourself.

If I had access to such a "bike mine" (aka gold mine), I would probably look for a bicycle with standard parts and would generally prefer a bike with some parts in top shape and others in need of replacement, rather than a bike with not so good parts. You might also look for parts (ex.: good wheels on a ****** bike or vice versa, or narrow road bars for your child). I would also avoid brands with non-standard parts such as:
- Peugeot (either French or standard parts depending on vintage)
- Schwinns (from what I read, non-standard wheel sizes, except for their 27").

But sometimes, these bikes are a good source of non-standard parts.