I'd go for the Frio if cooling performance is your priority.
The Zalman is a good cooler, but it sacrifices a bit of performance for quiet fan operation.The single biggest con is the fan isn't that powerful, but still cools very well, especially for the money.
I think the V6 has a balance of appearance and performance. but I don't have much experience with it.
The Dark Knight is a great looker and not a bad cooler. I have the original, and it isn't bad (holds my 2500k at 42c over ambient), but I would've gone for something else considering the price I paid.
The Hyper 212 is all about value - bang for the buck and does a decent job cooling. Not the first choice if you're overclocking, but you'd be hard pressed to find something that cools better for the money, especially if you pick one up on sale.
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