The safest and simplest answer, I think, is to simply donate to GiveWell for re-granting to their top charities. The folks at GiveWell are clearly "pros" when it comes to evaluating charities, so if you want to defer to widely-recognized experts, they're it. (And since they're so transparent, you can read through their research in detail if you want to reach your own conclusions.) You can also donate directly to their recommended charities, via their website. (Note that it's important to donate through GiveWell's website, or otherwise let them know about it, so that they can accurately track their "money moved", which is important for growing their influence.)
This is a "safe" option, because GiveWell's in-depth research offers the best guarantee available that donations to the charities they recommend really do get results. Further, by adding to GiveWell's "money moved", and hence influence, you can reap the (potentially much greater) benefits of promoting meta-charities. For these reasons, I gave $2500 this year to GiveWell for re-granting.
A more speculative option, with a potientially even greater upside, is to donate to GWWC itself (or its sister organizations within the Centre for Effective Altruism). GWWC is expanding fast, and has recently taken on some paid staff to help facilitate further growth. Given that each new member goes on to donate 10% of their income to highly effective charities, expanding the membership seems like a potentially very high-impact endeavour. Will Crouch makes the case:
Last March we did an impact assessment for Giving What We Can. Some more info is available here, and I can provide much more information, including the calculations, upon request. As of last March, we’d invested $170 000’s worth of volunteer time into Giving What We Can, and had moved $1.7 million to GiveWell or GWWC top-recommended development charities, and raised a further $68 million in pledged donations. Taking into account the facts that some proportion of this would have been given anyway, there will be some member attrition, and not all donations will go to the very best charities (and using data for all these factors when possible), we estimate that we had raised $8 in realised donations and $130 in future donations for every $1’s worth of volunteer time invested in Giving What We Can. We will continue with such impact assessments, most likely on an annual basis.
I encourage anyone interested in meta-charities to read Will's full post, and seriously consider donating to GWWC to help in their expansion. I donated $3000 to them this year.
So, those are the conclusions I reached this year. But I'd be interested to hear others' thoughts on where best to give. (Perhaps you can help me to find even better giving opportunities for next year.)
One final recommendation: Wherever you give, and however much you give, do so publicly. It may be too awkward to bring up in person (depending on context), but there's really no downside to social broadcasting via Facebook, blogging, or whatever. You can help your friends to think about effective giving, and by force of example help to promote more philanthropic norms. In such a way, the indirect effects of your giving may ripple out beyond merely the direct effect of your personal donation. That'd be pretty awesome.