Your questions are too subjective and you do not give enough info on what you have owned and ridden, or what style and materials you like, what kind of riding you want to do with the bike, or what the purpose is for the rebuild. There is no "best" other than one that fits you best, that you feel comfortable on, and is compatible with what you want to do with it. Two other huge factors: how much money do you want to put into it, how much time and effort do you want to expend on the project. As already suggested above, spend some time looking at the Classic & Vintage sub-forum here. Google up other bike forums/blogs.
Try to have some vision at what you want to accomplish and include that info with your questions. I did my first rebuild last year, a bike I have owned since it was new, 1992. I have now torn down and rebuilt 3 bikes. Some observations from those: the project will likely cost more than anticipated, you will need to acquire tools that were not thought of, you will make some mistakes and have some frustration at times, you will also have "ah ha"moments and feelings of accomplishment. If want to do this to challenge yourself, that is a good start.
A warning: this can be addictive!