Your dive knife is an essential tool for potentially life threatening circumstances and I take at least one with me on every dive. Gone are the days when divers took huge knives the size of your forearm to fend off sharks and today the knives on the market are more effective at what we actually need them for.
The material of the blade is probably the most important feature of the knife. There are two common metals and some others that all have their pros and cons.
Most of us are pretty familiar with steel, it's inexpensive, easy to resharpen and holds a good edge. The down side to steel is that, even marine grade steel, contains iron which will rust when exposed to water so you have to keep your knife clean and dry it in-between dives. To help prevent rust many divers use a hydrophobic gel to coat the blade so water doesn't reach it but you still have to look after the blade often to prevent rust.
Titanium is more expensive than steel but it cannot rust because it doesn't contain any iron. Titanium holds a good edge but is harder to sharpen than steel, it's also a lot lighter than steel so better for travel. If you're after a knife that you can leave in or on your gear with minimal maintenance go for Titanium.
The newest player on the field is ceramic which again wont rust and if you've ever cut yourself on a broken plate you'll know just how sharp ceramic can be. Right now they're limited to line cutters because ceramic doesn't have the same flexibility metals do but it makes for an inexpensive rust-proof line cutter.
Your traditional knife is like any other knife with a handle and a blade. They usually have a single straight cutting edge and a chunky handle. Traditional knifes are built for power and a serrated edge will make it great for cutting larger ropes. Your traditional knife is a good all-rounder than will cut anything but can be cumbersome or dangerous cutting gear off a buddy.
Hooks and Z-Knives have a trapped blade that are very sharp and good at cutting thin lines like webbing and fishing line. Because the cutting edge is protected it can be thinner and sharper so you don't have to apply as much pressure to cut. The cover protects your hands and equipment from accidental cuts but prevents you from cutting larger objects and ropes.
Shears are built like a traditional knife with a two-part hinged blade so you can use them as both a traditional knife and a pair of shears depending on your needs. Shears have more control than a knife if you need to do delicate work but are bulkier than a knife and can't cut thicker ropes so are good for fishing lines and webbing in an emergency.
FEATURES TO LOOK FOR
The tip of the knife can either be blunt or pointed. Pointed tips are good for fine controlled cutting but blunt tips are becoming more popular because they are safer and can be used as a screwdriver or prying tool in a jam.
Straight edges create a neat cut but can struggle with thicker ropes where a serrated edge can work much better. Serrated edges cut quickly but can be messier. Notches and hooks are good for small lines so you can trap it in the notch and only cut the line.
I haven't mentioned sheaths yet but you need somewhere to keep your knife safe without damaging your gear. Traditionally your knife was mounted to your leg as it was out of the way but this makes it harder to reach in an entanglement. Now there are plenty mounting options for all over your gear so you can mount a knife where you need one. Some knives can only be mounted to a limb so make sure your knife can be mounted where you want it.
If you do choose to buy a steel knife try to find one that can be easily disassembled for cleaning. Rust forms quickly in corners, edges and where the blade meets the handle. Rust can often grow unseen under the handle and weaken the blade, so your knife can look good but break when you go to use it.
So this is a hard list to create as each of these have a specific use so this is more of a top of each category than a ranking. Argonaut The Argonaut is made from a single piece of Titanium making it strong, light and rust-proof. The mount has a variety of options where you can mount it and the blade itself has a straight and serrated edge. Cisors Cisors act as a knife with a straight edge, serrated edge, cutting shears and pliers.
Trilobite The Trilobite is a compact and efficient line cutter with replaceable blades so you can rejuvenate your cutter whenever you need to. The size, mounting options and colour options make this an effective tool for any diver.
Mini The Mini knife is all you really need in the water. The compact blade isn't cumbersome, it has both a straight and serrated edge and the locking mechanism means you can keep it in a pocket or on a hose.