8# & 12# depending upon what you are doing.
For Tabata-style swings, use an 8#. Go on, laugh at me for being wimpy. Then go out and try it. After two minutes you should want to puke. Remember, Tabatas are four minutes.
The 12# hammer is good for pretty much everything else. 16# sledges are great is you can find them, but they tend to be a bit more elusive than 8# & 12#.