What's the best mulch?

Good question.
I've been thinking about this a lot lately, with the new guidelines on bushfire fuel load reduction.
Use gravel, the Victorian Government advises.
Thanks for that.
Gravel is a great mulch for level areas that are already planted. Absolutely. And it doesn't burn in a bushfire. Gravel and pebbles have also been very fashionable for several years (too fashionable, one might argue).
But for the 98% of us (number picked at random but I reckon it's certainly the vast majority) of us on sloping blocks or with developing planting, gravel is no use at all.
You can apply it to a slope all you like, but it won't be there after the next decent rain. It'll be in your drains or your garden beds or wherever you most wish it wasn't. It's also a pain in the butt to try to plant into gravel-mulched areas.
I don't know how to solve the problem of having combustible mulch near your house if you're in a fire-prone community.
We simply have to mulch, and often those plants that do need mulching, such as fruit and veg, are relatively close to the house.
Mulch keeps moisture in the soil and keeps it cooler on hot days and warmer in winter, hopefully replenishes nutrients, and reduces weeds, which also makes sure your plants aren't competing for water and nutrients. It also makes it a great deal easier to remove any weeds that do pop up (and they will).
Groundcover plants are probably the best idea, though not as instant as a layer of bark. They will burn - any foliage can burn - and they will require water from the soil, but they will always have a higher moisture content than dead woodchips and therefore must be a better option. Just don't choose plants with highly inflammable oils (part of the problem with eucalypts and pines).
If you have enormous water tanks, it might be possible to wet down combustible mulch on extreme fire danger days - certainly that should be part of your fire plan if your property is in danger and you are there to hose it down. That was the local CFA advice last season (prior to February).
As far as I know, there is no other fire-resistant mulch.
But fire fuel issues aside, what is the best mulch?

I can tell you what's not.
Plastic matting is not. It adds nothing to the soil and probably in fact damages it. And it's ugly. If you must use sheeting of some kind, invest in proper weedmatting.
I'm not keen on all that bright red recycled rubber, though it may have its place on roundabouts and shopping centre carpark edges. But it's not organic, and it's not helping feed the plants. It's recycling, which is better than nothing, but is designed only for weed prevention, not for plant support.
And after all, gardening is primarily about supporting your plants to do their best.
Anything that is taken from unsustainable forest practices should be out. Most purchased mulch is pine, but you need to be very careful buying anything like red gum or other eucalypt mulches. Ask the supplier - where do they come from? It's a pretty ridiculous concept to destroy native forests in one area so that plants in another place can be pampered.
But generally, bought mulches are by-products of milling plantation timbers or recycling green waste.
I'm not that keen on those blocks of coconut husks - they do suppress weeds but seem to keep water out of the soil. I used a few for the first year but the beds stayed very dry, even after good rain. The following winter I dug it all in, and can report that it does break up clay nicely, so not a complete waste. And very cheap.

DIY
You can make your own mulch, of course, with the help of your feathered friends. If you have chooks, leave the straw and manure to rest in a pile for a few months and it can go on your veggies and garden beds. You have to be a bit more careful with barn straw, if you have horses or cattle, as seeds in any manure can become weeds in a garden pretty quickly.
You can use grass cuttings and garden trimmings, though I would let lawn clippings die off in a pile somewhere before layering them on. They do get a bit slimy, so mix them through with other compost materials, such as dry leaves.
You can collect mulch, too, especially leaf litter from bushy areas of your block, which is great for indigenous plants. Leaf litter is certainly a lot safer in heaps around your shrubs where you can damp it down than scattered in the dry grass on a hot windy day. Rake it up, use what you need as mulch, and get rid of the rest of it before high summer - either in the compost (where it will take longer than other matter to rot down) or in your green waste so it can be re-used by others.
If you make compost, it's up there among the best mulches, although most of us can't generate enough to cover great swathes of ground.

For the veggies
Lots of people, including the good folk on the ABC's Gardening Australia, recommend sugar cane waste.
I don't. I used it a great deal in the first few seasons I was here, mostly because it's cheap, but it seemed to create an impermeable layer on top of the soil which actually prevented water from reaching plant roots. It might be good for breaking up heavy soils, but there are also different views on whether or not it adds nutrient content.
For veggie patches I much prefer pea straw. In fact, I love pea straw. I also love mushroom mulch. I could eat it. It just smells gorgeous and feels rich. It's gardener's chocolate. But I have to say it's more a compost than mulch. The only drawbacks with pea straw are that you do get a lovely random crop of peas, but just think of them as a wonderfully nourishing green manure and dig them in. Pea straw will also blow away in exposed areas. On the other hand, you will see it featuring in lots of local bird nests, which is kinda nice.
When making new veggie beds, start with a layer of cardboard or thick newspapers, and lay an inch or three of compost and then pea straw over the top.
I collect bags of oak leaves in autumn for the roses, but otherwise they seem to quite like the pea straw too.

Bulking up
For general planting areas, you need something more solid than pea straw: something that will look after itself, not rot down too fast, and preferably look good. That's why most people go for timber chips or bark.
Choose carefully, though. Some of it is quite recently milled pine, which is still too raw for your garden beds. It may even contain toxins that can cause serious damage.
It needs to look as if it's been laying about in a pile for a good few months before you put it anywhere near your precious charges.
Most bark and chip mulches actually leach nitrogen from the soil as they break down, so make sure you throw around some blood and bone (or pellets made especially for native plants) to compensate.
Most bark or fine chip mulches need to be replenished every couple of years.

On an angle
Most of my block, like many bush backyards, is sloping. It's possibly more dramatic an angle than other places, and I have done quite a bit of planting on the slopes. That's very common.
So how do you mulch on a bank?
On one relatively gentle slope I recycled carpet underlay and cardboard boxes and held them down with rocks. Then I plastered it with Eltham Mulch. Onion weed came through it this year but not much else.
I've used the Eltham mulch on even steeper slopes, sometimes almost vertical, and it just stays there. Never moves. I just chuck it down the hill (sometimes a tarp-full at a time) and it rolls down into place and does not move.
I love Eltham mulch even more than I love pea straw. It is dark pulverised bark mixed with thin strips of pale wood (pine, mostly) and it is especially made for mulching banks.
Now, I know it comes from Adelaide, and it's probably only called Eltham mulch because the Eltham Soil Shop gets it in especially, as it's perfect for the steep gardens around our way. Elsewhere, it's probably called Upwey mulch or Macedon mulch. It's fabulous.

How thick should mulch be?
The opinion on thickness of mulch has changed a bit over the years - experts used to recommend 10 centimetres (which is a bloody lot of mulch). Now it seems like most recommend about 5 to 7 centimetres. Any thicker than that and you risk preventing water from getting to the root zone.
A rule of thumb is the bulkier the mulch, the thicker you spread it. Fine mulches like compost and lawn clippings can be 3 centimetres.

Wherever you are, shop around for the right mulch for your block and don't be afraid to ask the suppliers what it's made of and where it comes from.