At this point, it is too early to tell who will run for office. But in general, one ought to choose someone from the more libertarian wing of the Republican party. I would find a libertarian score ranking of candidates and use it as one of the tools to make a decision.

Republican Liberty Caucus has a score ranking of Republicans. Take a look: http://www.republicanliberty.org/libdex/li2004_over.htm
Keep in mind that some scores may be misleading. For example, Sam Brownback may be too socially conservative. As far as McCain goes, he has long time connections with Vegas interests, so he is clearly not our choice.