Personally here is how I rank them and I think there'd be general consensus.
Crash tested -> custom metal/welded -> plastic -> aluminium -> wire -> seatbelt
I really want to get a Gunner Kennel at some point I believe it is the only one that has passed every single kind of test including third party tests. The Ruff Toughs are the second best on the market, and I would consider them to be the best alternative to a Gunner where a Gunner might not be feasible.
Crash tested crates are the best option on the market specifically because they have actually been submitted to crash tests as well as other extreme situations. The Gunner Kennel is the only crate on the market capable of accommodating large dogs that has been rated and certified by the Center for Pet Safety, a registered non-profit. It has been submitted to some extreme testing by Gunner themselves and has also been field and crash tested by others. It's expensive (although not that much considering, and they do offer payment plans). It's actually cheaper than a lot of the custom and welded metal crates. It's the kind of crate that is so solid your dog would still be safe traveling in the bed of a pickup truck with it. They also make their own downties to secure the crate to the car/truck which have also been crash tested (and would be the only ones for crates on the market to have been certified). The main drawback is the sheer weight of the large ones, they are definitely not easy to load and unload regularly out of the car. The size is also a limiting factor, their largest model the G1 Large is just a little under 41 inches in length which means the inside/usable length is much smaller, given so much of the outside length is due to the double layer/shell of it. For comparison Nadia was travelling in a 36 inch ordinary plastic crate until this spring she got upgraded to a 42 inch. Given that she spent time in the crate not just on the road but during training days could be several hours resting in it between her turns and her bathroom break, I wanted something a little roomier - she couldn't quite lay down in a relaxed position with her front paws fully extended. Now with her current size she can comfortably stretch and much more easily move around. I'll have to measure the floor but I bet the Gunner's usable length is smaller and I may have to consider getting a Gunner in addition to my plastic (petmate I think it is) one, not instead of it. Although people are saying their German Pointers and German Shepherds fit in it, I wonder how comfortably they actually do fit. And we don't have to worry about things like a tail to include in length!
The Ruff Tough is not certified by the CPS but is a popular alternative to Gunner due to being cheaper and much more lightweight so easier to use as a normal plastic crate. It is supposed to be escape proof and dog proof, however where I've seen Gunners survive some serious crashes with totalled cars and nary a scratch, I've seen anxious dogs damage Ruff Toughs from the inside. Another advantage over the Gunner is that while the largest Ruff Tough is the same external length as the largest Gunner, there are several more inches of floor length in the Ruff tough than in the Gunner. There aren't any other crash-tested crates (as in, tested by third party researchers or certification centres, not just tests at the manufacturer's own lab) for large dogs, and Gunner is the only one certified by CPS.
Gunner videos:
https://pack.gunner.com/videos/
(Take a look at one of the CPS videos, the Gunner Kennel vs a 630lb Sled which also features other types of kennels and the Indestructible Dog Crate testing which includes bullet test).
Ruff Tough video:
https://www.youtube.com/watch?v=NJUD4tM_U44
Custom Metal/welded ones are popular with people in the working dog world just because they're a lot harder for a dog to break out of (although the Gunner does just as well there) and since they can be custom made for the dimensions of your car, they maximize the efficient use of space. However the downside is they aren't easy to get in and out of the car if you only have one vehicle and need to switch things out to put something in there. Most of them are not crash tested but are solid enough that they'd likely resist most crashes. Depending on the model they also tend to have wider, vertical only bars so they offer more visibility towards the dogs if you have a dog that needs to be consistently checked on.
Plastic crates. These are the airline style varikennel or varikennel inspired models. This is what I currently use both for Nadia and Vanya. Fairly affordable, can come in multiple sizes. I like that they are easy to get in and out of the car as needed, they're lightweight, easy to clean. My grandmother uses a Giant size one for her standard poodle in her own car. They're easy to secure. They likely wouldn't survive a serious crash, but in most cases would manage to keep the dog contained even if the crate is damaged and/or offer a chance of survival to the dog in the even of a crash. For a crate-trained dog who does well in the crate they do the job just fine of containing the dog, although some models have different latches of varying complexity, and the option to add padlocks if you have an anxious dog or escape artist. I find them to be safer than the wire crates or aluminium crates, simply due to the fact that besides the door there is generally no metal, so if the crate does break in a serious crash, bends or tears, there aren't any pieces that the dog could seriously injure themselves on.
Aluminium crates. Usually much more expensive than plastic, and a lot less safe to boost. The Impact Crates are all the rage for example but for anxious dogs and escape artists that chew their crates I have seen some seriously sharp bits of metal sticking out, sometimes a piece that is directly ripped up from inside the crate. The way they'd crush, break, or tear in an accident might leave more hazards to a panicked, trapped dog than a plastic. The impact crates do have a collapsible line I think so useful for storage and bringing them in and out of the car. They're also perfectly flat on top so easy to stack, and useful at a show to double up as a table. And they come in fun colours. Nonetheless I find them very expensive for what they actually are in terms of safety or protection.
Wire crates offer the advantage of visibility (highest of all) if for example you have an anxious dog, or a dog that wants to eat their bedding, or a dog that is prone to getting car sick. They're also generally fairly cheap and they're collapsible. Easy to take in and out of the car and travel with. They will of course contain the dog in most smaller accidents. The main drawback is that in the case of a serious crash or accident, there is a much higher risk of breaking, and like the aluminium crates, sharp metal bits that could injure or kill the dog. So it secures the dog and prevents the dog from turning into a projectile, but doesn't protect the dog as much.
Seatbelts... Seatbelts are not very secure for dogs unfortunately, especially for larger dogs. Tests were conducted not that long ago and there is still a risk of the dog turning into a projectile either from slipping out of the seatbelt, or the seatbelt breaking. There are also increased risks of serious or mortal injuries to dogs from them. It is almost preferable to use a normal harness and secure it to the floor of the car.
Currently there are two brands of harnesses and three models for large dogs that are CPS certified.
ZuGoPet - the Rocketeer Pack
Sleepypod Clickit Sport
Sleepypod Clickit Terrain
CPS website:
https://www.centerforpetsafety.org/
(check under CPS certified to see the individual videos for the tested products)
Happy shopping and safe travels!