I just bought the Game of the Year version second hand and it was the best purchase of my life. Words can't describe how much I love this game.
The dialogue, the combat, the setting and surroundings and sidetrack- All of it is phenomenal and I just love it!
It's incredibly engaging, with the main quest, to the DLC (Both Undead Nightmare and the others included) the voice acting is natural and although the game was released 15 years ago it looks absolutely incredible now. There's endless things to do, and although there is a main plot, you don't have to do it necessarily past the tutorial. It's amazing and I will always recommend this game to anyone.