I like to use one-shots as pickup games when members of the group are absent and the regular campaign wouldn't really function properly with said PC missing (which is almost always the way I run campaigns).
So I use simple systems, like Mausritter or my absolute new favorite, Mothership! Systems like those means it takes but a few minutes to make a PC, so very little time in the session is dedicated to character creation.
The throwaway nature of the PCs means a couple of things that I think really enhance the experience over a regular long form campaign. With a one-shot, because players know it will probably only be a single session, they tend to be WAY less risk adverse, which means PCs take all kinds of crazy risks that don't normally happen in a campaign where the players try hard to keep PCs from harm. It makes for some interesting and unusual gameplay, and lots and lots of laughs. I really love Mothership especially because it's a horror game, so a TPK at the end of the session is pretty much expected. Makes for a nice little concise story with a satisfying horror movie ending.