For
The Witcher (Enhanced Edition) and
The Witcher 2: Assassins of Kings (Enhanced Edition), you receive everything as
olnorton mentioned. They include all of the official patches, gameplay improvements and graphical touch-ups that the games received. As for The Witcher 3, I definitely recommend going for
The Witcher 3: Wild Hunt (Game of the Year Edition) as it's the complete, definitive way to play the game. It gives you both of the expansions,
Hearts of Stone and
Blood & Wine, both of which serve as an awesome outro to the story of the game overall, adding somewhere around 50 hours of new content; even more if you take your time with them. It also automatically includes all of the free DLCs that CDPR released, as well as some seriously awesome goodies. Get the Enhanced Editions of 1 and 2, as well as the GOTY Edition for 3 and you won't have to worry about buying anything else if you don't want to.
That being said, I'd also recommend checking out
Thronebreaker: The Witcher Tales and, if you really like the series,
The Witcher Adventure Game. The latter is a digital adaptation of the Witcher board game and can be a bit of fun on the side (includes a multiplayer option, but I haven't played it with anyone yet), while the former is a medium length narrative-driven RPG with combat based upon the optional Gwent card game minigame in The Witcher 3. It's a lot of fun and well worth the purchase, especially if you enjoy the other Witcher games and want to support CDPR.
Oh, and if you're interested in Thronebreaker but aren't sure if you'll like the combat, either play Gwent in The Witcher 3 or give the free
Gwent: The Witcher Card Game a try. Either will give you a taste for the combat in Thronebreaker but even if that combat isn't your thing and you want to play the story, CDPR gives an option in Thronebreaker to skip any battles with automatic success. This is great for me as I am particularly bad at Gwent, meaning I can just focus on the narrative instead of getting frustrated with loss after loss.
In general, I highly recommend buying all of the Witcher games on GOG. Not only are they fantastic games but GOG and CD Projekt as a whole, they deserve every bit of money we give them due to the way they genuinely care for their players.
