I use the Novac room. It has a Mojave Express box, two vendors, a doctor, Workbench and a Reloading Bench within walking distance, plus merchant caravans passing by the road every now and then. But its location is quite far from where most of the action is in mid- to endgame.
Victor's shack is probably a close second. Three vendors (albeit lousy ones), a doctor, a Mojave Express box, a Workbench and a Reloading bench within walking distance. The only downside is that the Mojave Express box is a bit of a long walk from the shack, which is tedious when over-encumbered