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The one thing that can be said with certainty is that "goodness" in an embedding resin is to some degree in the eyes of the beholder!
But the listing of possible resins can be segmented in several different ways, by chemistry for example, epoxies vs. acrylates, or conventional multicomponent kits vs.the new "one step" kits. Or they can be segmented as we would prefer as being in terms of their water solubility:
Not "soluble" at all (requires complete dehydration):
Partially "soluble" (requires a partial dehydration):
If a sample can be safely dehydrated, then in general that is more often than not, the best first approach. The SPI-Pon™ Embedding Resin Kit or the SPI-Pon™ Araldite Kit make good first choices. If a lower viscosity resin is needed, then the Spurr formula Low Viscosity Kit should be considered.
The just given suggestions are based on the sample being one that can be completely dehydrated with alcohol. If one is doing immunohistochemical type work, for example with immunogold, then it is generally advisable to not use a protocol requiring a complete alcohol dehydration, but one should be using a system that requires only a partial alcohol dehydration.
For immunohistochemical type work, where the preservation of the sample's
antigenicity is the highest priority, one of the most widely used kits for
this purpose is the Lowicryl™ series of resin kits. However, the
SPI-Chem™ "low acid" HPMA is giving many users outstanding results using an
essentially similar chemistry but for a significantly lower price. In
addition, the SPI-Chem Low Acid HPMA is available in the 
form which permits a much lower cost for
international shipments.
The one-step resins, including LR White™ , Unicryl™ , and Lowicryl Monostep™ offer the user convenience with results reported to be virtually as good as what can often times be obtained with the previously mentioned kits.
Some samples have antigenic sites that are so fragile that any alcohol dehydration is thought to be deleterious. This is where the SPI-Chem Low Acid GMA is recommended. While there are other GMA based systems being offered, the SPI-Chem™ version is truly "low acid".
The SPI-Chem Low Acid GMA is often times indicated when the lowest possible viscosity material is needed for difficult to infiltrate materials. The viscosity at room temperature is a bit lower than water itself. Histologists using the SPI Chem Low Acid GMA are often times surprised at the extremely low levels of background staining that are present (because of the low acid nature of the GMA).
Certain "materials science" samples that can not be safely dehydrated, for
example "soft lens" systems and also, reverse osmosis membrane systems (or
any ultra filtration system) which can not be dehydrated without collapse of
the "pores" can be easily embedded in the GMA system and polymerized.
Because of the water solubility there is the possibility of precipitating
silver chloride into the pore structure so that once embedded, thin
sectioned and examined by TEM, the silver chloride acts as a decorator of
the pore structure.
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