That depends on the price levels. At the moment I think the best bargains are the low-div variable-rate series (FNMAO, FNMAP, FMCCG, FMCCI, FMCCJ, FMCCL, FMCCM, FMCCN), but that's only if you don't really care about liquidity.
There is an evidently-unable-to-be-named Twitter account that addresses just this question regularly.