If you just had to pick one in the spring of 2011, your best bet would be github as it seems to have the most mindshare. Moreover, in the days of this whole internet thing, you can easily host it one place and promote things across the board.
I'd generally argue for:
a) Pick the best source control/issue tracking/wiki option you can find for your tools and your workflow. Software is hard enough, real jobs suck enough, no reason to use bad, old tools to make stuff.
b) Second, pick where it makes sense. For example, if it was a .NET project you almost certainly at least want a codeplex presence as that is where people look first for .NET OSS. If you are writing a ruby mixin, maybe rubyforge is your huckaberry.
c) Market the project wherever it makes sense. Absolutely no reason not to spread across social media these days -- it is pretty easy and it makes sense to have the flag everywhere.
But, end of the day, if you are making good, useful software that people need, they will find you in most cases.