I'm a huge fan of the Witcher, since I grew up on Sapkowski's books and the hilariously bad polish TV series based on the short stories. So when first Witcher game rolled out, I was super excited. And most importantly, I wasn't disappointed after playing it. I've played the game at least ten times since it came out, probably more. If that doesn't spell "great game" to you, I don't know what will.
The combat system is a love or hate relationship. I personally love it. What a fresh change from usual clicking like a mad-man, Diablo (or Witcher 3) style. Sure, you gotta learn how to use it, but at least the game challenges you on that front.
I love the potions and oils making, something that was really missing from following Witcher titles. Again, you gotta work towards making anything in alchemy, which makes the whole thing more believable. The skills tree was perfect, if maybe a bit unusual, but it's again something I missed in following Witcher titles. So on the front of skill trees and alchemy, I'd say first Witcher is far better that W2 and W3.
The environment, especially what you're allowed to do, gets a lot of hate, but I'm kind of 50:50 on that front. The game doesn't flaunt it's an open world and it's not. The fact that you can't jump or get to places you see, unless the devs made a path, is kinda annoying, but it's just how it is. On the other hand, you visit so many different, beautifully made areas, that you surely won't feel bored by your surroundings. If you don't expect freedom Witcher 3 or Skyrim style, you most likely won't be disappointed.
The story is pretty great and fits with books rather nicely. However, it is kinda obvious that devs didn't really believe in making more Witcher games after this one. Or rather, they changed the course of the story after they made this game. But if you haven't played this game already, it can explain a whole lot of things happening in W2 and W3. I just wish the game treated its female characters better.