If you're talking about taste(which samples we prefer...)...
that's a question of... well... taste. But really, sample whatever you wanna, it's your beats. If they sound tight, well, good job. And actually, it's always nice to find a sample somewhere no one (or few people) use to check out.
If you're talking legal stuff, well, when you use a sample and want to use your beat commercially (making money off of it), you'll have to deal with copyright laws. The orginal artist who made the track that you sample from wants to have a cut of the money you're making. Of course, sampling from a no name is less expensive than sampling from Michael Jackson.
Do a search on the topic, you'll see that this has been talked about a lot here...