There isn't a single definitive answer to this because many of the options have variables that affect the properties of the set/hardened product. Obviously this includes any blends of an adhesive with a wood dust or other powder, but it's also relevant to straight epoxy products and other two-part fillers where there is a separate hardener — in some products only slight variations in proportions can greatly affect final strength (usually negatively).
So in no particular order I'll list some very strong fillers that can be used interchangeably for some jobs, but not all.
Bondo and similar
Bondo, and other polyester fillers, are the go-to product for many people working with wood where strength is the requirement. They are easy and fast to mix, bond very well to wood, harden quickly and dry extremely hard1. On the downside they have a characteristic sweet plastic odour (you're smelling styrene) that some people find objectionable and is toxic at high concentrations.
Basically only the one consistency is available to the user.
Epoxy + filler
Where a combination of adhesive properties and filling are required the choice is going to be epoxy + a dry filler material. Filled epoxy is extensively used in boatbuilding so its strength and resilience is well proven. The filler used is chiefly wood dust but there are other filler powders possible as well as some additives, see Large hole filler products, what is available
Filled epoxy mixtures can be varied greatly in consistency, obviously by using more or less filler but some epoxies are more viscous and others much more liquid so the starting point can vary too.
In order of increasing thickness the consistencies that seem to be most used are generally likened to ketchup, mayonnaise and peanut butter. See this page on the West Systems site for more info.
Epoxy putty/wood filler
You could think of this as the factory-made version of the above, although it's not really similar to many DIY mixtures and you have just the one consistency to work with. Also, these generally set quickly, often in under 10 minutes and sometimes as quickly as five (faster in warmer conditions!) so they can't practically be used for large repairs unless you work layer by layer, or in sections.
These are usually sold in 'Tootsie Roll' form, where one component is wrapped around the other. This is a major weakness with all fillers sold this way, because the two parts are already touching obviously over time this starts the hardening process in the resin. So even sometimes in product you've just bought2 when you blend the putty you'll find hard lumps that are annoying at least, and bad enough that you can't continue to do the repair at worst. DAMHIK -_-
I would recommend this sort of thing only to someone who does a lot of repair work so will use it up fairly quickly, and can buy from a source where there is a fast enough turnover that you never get old stock that has sat on the shelf too long.
Durham's water putty
If you're in North America this is one other product worth mentioning that I don't think is sold in most other markets. Durham's Rock Hard Water Putty™ to give it its full name may be the oldest option on the list since it has been on shelves since before WWII.
It also dries very hard, and again perhaps a little too hard for some applications.
Durham's comes as a dry powder that must be mixed with water to form a paste or putty. Multiple consistencies are possible by simply adding more or less water, but note it appears to be strongest when mixed close to the default water:powder ratio — 3:1 by weight.
1 A little too hard for some purposes. Bear this in mind if working with softer materials (most softwoods, softer hardwoods, MDF, some plywoods) where you need to sand flush with surrounding surfaces.
2 You often can't tell how long it has sat on a shelf before you picked it up.
