Which vintage pens are best for daily use?
If one is sufficiently careful, nearly any vintage pen can be used on a regular basis. Nonetheless, most pen lovers avoid using particularly rare and delicate models, such as Parkers in Mandarin Yellow and most pens in red hard rubber (Waterman lever-fillers are an exception, since the material used was layered so that the embrittling red colorant is mostly on the surface). If you want to be able to use exotic boutique inks, be aware that they may not be fully compatible with all vintage pen materials -- read more about ink selection here.
Look in participants' pockets at a pen show, and you are likely to see a lot of Parker 51s. These are among the most robust of classic pens, but their precursors the Vacumatics and the Duofolds are equally practical for the user. Lever-fillers by Waterman, Wahl-Eversharp, Conklin, and Sheaffer are also very practical pens, strongly built and easily serviced. Other brands to keep in mind are Mabie Todd (Swan) and Conway Stewart -- though many other marques are equally suitable. Don't be afraid to ask for advice when selecting your perfect vintage pen!