Nine years have passed since the original (and incredibly broken) version of The Witcher first appeared on the market. Back then I considered it to be a pile of garbage as the whole thing barely functioned, but instead of simply running away with our collective money CDPR decided to take the impossibly hard route of actually fixing all of The Witcher's problems. As you're probably aware given the presence of two sequels and numerous pieces of DLC - they have managed to not only turn The Witcher around, but also spawn an entirely new series with millions of fans!
If you're never given any of The Witcher games a try, but would love to do so if the price is reasonable, you're in luck as the whole series is currently on a rather hefty discount on Steam. Do bear in mind, however, that the sale will only last until October 24, so don't spend too long slacking around or you just might miss it.
As for my recommendation, I'll just quote what I've said during a similar sale back in August given that my opinion has more or less remained unchanged:
"I would probably start with The Witcher 3 or 2, depending on your budget. And don't worry, you don't have to play any of the games in the series in order to understand what's going on, they all tend to have fairly well-contained stories. That disclaimer aside, the reason I would recommend these two over The Witcher 1 is because they feature your standard action-RPG mechanics and combat, things you've probably seen in numerous other games, so they should be easy to adapt to and enjoy.
However, if you don't mind button-mashy combat and an utterly terrible introductory section then I would also recommend you play The Witcher 1. In my humble opinion it contains the best overarching story in the entirety of the series, with the end "twist" being so mind-blowing you're going to want to replay half of the game in order to see if all of the pieces fit together... and before you ask, they really do!"
As a final note, allow me to just say that these are some of my favorite games of all time, though I might be slightly biased here given how much I love a good blend of storytelling and action, so if you harbor even an inkling of curiosity - I would heartily recommend you go for it! Have fun!