Kalameet
Now days most good villains don't get a boss fight (or at least a good one), it's all cut scenes and quick time events (I'm looking at you 343i). Kalameet, while he has no dialogue he does have an interesting background and offers one hell of a tough boss fight (it is Dark Souls after all).
A close second would be Saren, while I'm not a huge fan of Mass Effect or even Bioware, I do like Saren. He ticks the boxes that make a video game villain great.
English
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I'm still working on Manus, afterwards I'll face Kalameet.