The plots I enjoyed the most from all 3 games were the ones that took the player to a variety of locations and places that created a beautiful atmosphere with the music, sound, and visuals alongside ones where Geralt connects with people important to him. The northernmost areas of the game, around Kaer Morhen and the isle of Skellige were the most fascinating. The numerous quests around the Islands and the fresh outdoor open road feel were invigorating in the way that reading a book took my imagination into a different world. The massive outdoor world is better than any other game I've seen, but without the cityscapes like Novigrad, Vizima, and Oxenfurt, I'd be less appreciated. I enjoyed that massive long plots were in both cities and on the road in the third game, and how you could easily swap between them.
In the first and third games, the music and scenery at Kaer Morhen (particularly the music in the first game for the keep) just made me want to wander its empty halls or chat with each character and spend some quality time with them. It definitely captured my imagination right from the first game. I think the first game has some of the best music.
Too many characters you meet are ugly generic caricatures, whereas only Geralt himself and the sorceresses, and hookers are attractive, to the extent the 'eye candy' aspect of the game is far too overdone alongside the ugly personalities and appearances of most common characters. This is the worst in the first and second games. The third game has more characters who don't feel driven to quite the same extreme in terms of how unpleasant they're meant to be.
The best quests and plots were ones you do alongside the Witchers, Triss and Yennifer, Ciri, Regis, Shani, Dandelion, and Zoltan.
Second to that, ones involving prominent figures or unique characters like Foltest, Adda, Kalkstein, Saskia, Roche, Iorveth, Olgierd von Everec, The Bloody Baron, Emir Var Emhrys, Gaunter O'Dimm, Crach an Craite and his kids, Duchess Anna Henrietta etc. etc.