I have found that the addition of a single drop of amber medium, to a pea sized paint nut will dramatically alter the properties of the paint. If one mixes one drop into the paint nut, and then waits one hour, the paint will stiffen up, and perform more like a jewel paste, than a loose oil paint. This will permit many impasto effects, as well as multi-layered techniques, all on the same day. The strokes will tend to remain exactly as they are laid down, with no slumping, fusing, leveling or changes of any type. Some copals permit this, but in a slightly different manner, and I know of no balsams and certainly no dammar containing mediums that will perform this way. Drying oil alone will absolutely not permit these effects.
At lower resin to oil ratios, the medium will permit some effects that are very similar to the glass like, or enamel appearing effects that one may observe in the works of the Flemish masters.
Is amber the best of all oil painting additives? Quite frankly, I would say that there is no one medium that is the best. The better question is which medium is best for which special effect. In that regard, there are certain special effects that amber will permit that would be exceedingly difficult, using any other medium. Does it duplicate the effects of mediums containing larch turpentine, or Canada balsam? Absolutely not. It is important to ask what one is trying to accomplish. Many artists take up oil painting, and then go looking for a medium to help them out, without knowing what it is that they even want to accomplish, or what special effect they are after. This is a flawed, and costly method of progressing.
If oil paint alone, or with the addition of a small amount of drying oil, is all that is necessary for an artist to carry out their work, then certainly there is no need to throw amber, or any other medium into the mix. If on the other hand one is looking for effects as stated above, it may be a nice addition. I don't recommend any particular medium, but after having recently made and used a batch of Maroger medium, if I could only use one medium, in addition to linseed oil, it would be Maroger. Although it doesn't perform the same as amber, it has a quality that is hard to describe with words, other than to say one has to try it for themselves, in order to understand. Still not saying that any one medium is the best.