lowyhong: That seems to be the general consensus, which is odd because I like it. I'd even go as far as to say Clarence is one of the coolest villains I've ever encountered in a video game.
Anyway, OP just go with both. You can't go wrong. Black Plague is at times scarier than Amnesia, though as far as atmosphere and tension go, Amnesia wins hands down.
I agree with the first part. Clarence's voice was a bit silly, but it felt right. I thought it was better than Overture by far, though. It was creepier, there was no combat, and the story was much more interesting. I still hate Clarence after that one incident, though...
As for the second part of you post, I think Amnesia wins in the scary department. The first time I played it with headphones; I am currently playing again without headphones, and I really wonder how on earth I ever played it with headphones.
But to the OP, here's my opinion: Get Overture for a good story and creepiness. Get Black Plague for a great story and more creepiness. Get Requiem for complete and utter wierdness, creepiness, and slightly more challenging puzzles. Get Amnesia for all of the above, plus outright crap-your-pants scariness and if you feeling like being disturbed.