I have to agree with the adventure games mentioned. Those are usually the best bet for a good script and good story. Most of the Lucas Arts and Sierra games were good. I would also recommend Beneath a Steel Sky by Revolution. It's a typical 2d adventure game that takes place in the future in a massive city. The writing is superb I thought. As for other genres, I found the Wing Commander games 1-4 had excellent writing and dialogue, especially the 4th.
I have to recommend Giants: Citizen Kabuto as well. The storyline isn't that great, but the script is hilarious. That game is truly one of a kind, the humor is very dry and witty.
Arcanum also has good dialogue if I recall.
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