When I was first starting out years ago I would have told you Soul. If you are just starting out, that probably is the best genre to start with, because honestly sampling entire soul songs is not that difficult. You will get some incredible arrangements, soft drums typically, and a lot of different instruments to sample all within the same track. Now as I've gotten a bit more crafty I would tell you sample anything and everything you find to be dope. I'm to the point now where I use my turntable and vinyl almost like a VST in the sense where I'll have my beat almost done before I start looking for certain instruments to sample...that's when the real fun begins because at that point you can just about sample any record if the musical key matches.
Jazz would also be another genre as well.