Rod Serling wrote the vast majority of the tales, along with Charles Beaumont and Richard Matheson. Some were original, some were based on short stories written by others, and the vast majority are, at the very least, good.
In fact, the biggest issue that Twilight Zone has is that it's entered our collective consciousness - so many of the stories we know the ending of before we've even seen them - and even the DVD intro menu has spoilers. But it's to the show's credit that even knowing the ending of some stories fails to dull the punch. On top of that, there are still so many good and great stories that we've not had spoiled, that it can still regularly surprise, even when you're looking for the twist.
Some of you will have had this on your 'one day I'll watch such-and-such' list. Well the show's been around for fifty years, those other newer shows can wait a while, make a start.
For my non-spoilery review of the first season, go here.