I second that. At least as far as 3rdPP cRPGs go.

The only games I'd suggest as being similar to the Witcher would be Gothic 1/2 and Risen.
They're also single character, 'dark fantasy' rpgs with similar focus on realism in combat.

For the rest:
Torment & Bloodlines -- RPGs with well written and interesting stories.
Divinity II -- Highly enjoyable lighthearted action rpg.
Two worlds -- Mediocre action rpg. Avoid.
Amalur -- Huge open world action rpg. I actually preferred this one to skyrim.
Fable -- Avoid, period.
Alpha Protocol - Unique game. Probably most interactive storyline ever.
Bioware rpgs - If you've played one you know whether you like them or not. If you haven't at least try one, start with the best (baldurs gate).
Drakensang - Great games (I prefer to bioware recent stuff). Traditional rpg mechanics and setting, beautiful visuals, let down by weak story.

this this this this.
Id be hesitant to call it similar to the witcher.. but its one of the very best in the genre.

The Witcher is really unique. I don't want to say is the best, but it's really original.

From visual point of view, the Gothic games are similar, also in combat style. Gothics are really good games, and now fans have fixed many bugs, they are enjoyable. But they are developed in a open world, not like The Witcher. Drakensang is developed in episodes, but has not the cultural complexity of The witcher. From this point of view Planescape Torment is more similar. I add also Geneforge.

I should add as a counterpoint, I enjoyed Fable 2 at the least, and other friends have raved about 1 :P no knowledge of 3 though

If you want to try fable I now is the time. It's 2 dollars on steam.

An interesting but relatively unknown game I would recommend is Venetica.

I second this wholeheartedly. As Deus Ex being one of my favourite games, I really liked the gameplay of AP.

I only played it until I was close to the final chapter. The industrialized Venice in the last chapters wasn't my taste although the first chapters were much to my liking.

Try the total conversion: The Bitcher.