IMHO - For best epoxy performance, need to pick proper material for job.
Brands like; West Systems, System Three, and MAS produce many different types of epoxies. Mentioning a name is like saying a single auto maker is great at making every kind of vehicle; which is never true. But when using the RIGHT type of each brand, you will succeed. They key is using right epoxy for job.
Most of the retail brand epoxies sold in volume (quart/gallon) above are laminating epoxies. They are intended to be applied in thin films (< 3/32"), and used for bonding fabric to wood/paper. They work well for wood to wood bonding, but will not be strongest glue joint possible. Need to use a structural epoxy for highest strength in critical load applications. But since wood tends to fail before glue joint; few know or care if the epoxy being used is 'best' for most applications?
Only if using hardest woods in high stress design (or in wood to metal applications;, will average user see the difference in bond strength and need to use structural epoxy.
The challenge with structural epoxy is they are designed to reach peak strength with defined thickness bond line, using a long slow cure to minimize cure shrinkage related stress. Most epoxies have similar strength with minimal 0.002-0.010 thick bond line. It is when you have rough surfaces, and need to fill a 1/16 gap that you really need a structural epoxy system. As mentioned above System Three T-88 is one such material. If need to quantities larger than sizes sold at retail; folks at Ellsworth adhesives carry bulk Loctite and 3M structural epoxies used in commercial operations.
Typical laminating or structural epoxy mentioned have one serious issue: Thick section use or gap filling.
When need to fill any gap over 1/8" regular epoxies can self heat; which causes thermal run away and drastically shortened cure times. Another defect with thick section cure and excess heat is darker amber tone as amine curatives overheat. So this reason, it always best to use the slowest curing epoxy your application can tolerate. If you can afford to wait 24hrs for cure with slowest epoxies, you will be rewarded with lower stress joint, and typically less color change.
So what to use?
IME - Best epoxy for filling large 1/4"+ holes/voids/cracks in a casting type epoxy. When filling smaller defects, just about any epoxy can be used, as long as you don't mind amber color after cure. If you want clear fill, have use special clear hardener or epoxy designed as clear epoxy system. West Systems #207 hardener is clearest version they offer. Many other companies sell clear table top resin systems that work well for crack filling slabs, Just don't exceed the thickness recommendations.
Note about coloring epoxy:
Best way to color epoxy is with powdered pigments, or a pigment dispersion. Pigments help hide (but don't remove) the amber tones from curatives. There are tons of online suppliers selling epoxy pigment powders. The folks at Ecopoxy are one source with wild array of colors and metallic effects. System Three offers pigment dispersion that are easy to use.
If you want to be cheap, dried coffee grounds or dash of toner from a spent printer cartridge works as it does not take much powder to color epoxy. Can also use standard acrylic paint (artist colors in tubes, not kids paints in bottle) to color your epoxy resin. Add a max of a pea sized dollop, for every couple ounces of resin. Be sure to add the pigment to the resin only, and get it mixed in well; BEFORE adding the curative.
If you do use an excessive amount of acrylic paint, if will soften the epoxy. But when colored epoxy is used as wood filler under a top coat; no one will notice. Epoxy will also soften slightly when using solvent based dyes/tints; as the solvent carriers are glycol that don't react with epoxy, and take days to evaporate. Liquid tint/dye (like Transtint dyes) will also change based on curative color, and shift with amber tone; and are not best epoxy coloring choice.
Hope this helps!