Which Scuba Fin is best for cold water?
You will need open healed fins for cold water because in cold water it is necessary to wear boots or a drysuit. Fins that fit over bare feet are only suitable for tropical waters.
In terms of types of fins there are basically two choices. (a) paddle fins that have a single wide blade and (b) split fins that have two narrow blades.
Since you asked this question on an internet forum you'll probably get people recommending what they own and recommending against what they don't own. The truth is, both types work. That said, for advanced finning techniques in which the diver swims backwards (yes, this is possible), split fins would not be my first choice.
In terms of giving you a base line I personally think (even though I do not own a pair) that the "Mares Avanti Quattro" are a good baseline for comparison.
If you are wearing a drysuit then heavy rubber fins like the Scubapro Jet fin (or something like it) may be useful because they have negative buoyancy and will help keep your feet down without the need for ankle weights or something to that effect. Many technical divers use these types of fins as well, in part because they are quite suitable for swimming backwards.
I wish I could recommend a good split fin for a baseline but I don't have any experience with them so someone else will have to help you on that.
Now my question to you. Why do you want to know this?
R..