In The Witcher series of Netflix it is seen that Yennefer is sent to Aretuza to learn magic. She never performs exceptionally in any of the trials. She is neither able to float the stone, nor catch the lightning in the bottle but still is considered one of the best students after her training is over. How is she so good of a sorceress?
In Episode 2 "Four Marks", Yennefer creates a portal to Aretuza without any training.
I assumed that creating a portal is more difficult than floating a stone. Furthermore, Tissaia De Vries comments on the fact Yennefer has lost a lot of blood, due to her suicide attempt. This sounded to me like Tissaia believes Yennefer is capable of lifting the stone and has put her failure down to her blood-loss.
Later, Yennefer fails to read minds, but she gives up too early. This fact is explained later when Istredd states the same task took him three days. This interaction highlights Yennefer's impatience.
Yennefer fails to catch the lightning in the bottle, but instead catches it in herself - her own body a vessel for the most potent chaos. Shortly after this encounter, Tissaia reveals she believes herself and Yennefer to be of similar temperaments - allowing feelings, impatience and anger to control her, rather than controlling them and chaos with it. I understood this to mean Tissaia sees something in Yennefer that she doesn't see in the other students. Perhaps it is favouritism, perhaps Tissaia is projecting her experiences onto Yennefer... or perhaps Tissaia sees something in Yennefer that we cannot see and/or are not shown.
Either way, as stated by Tissaia, magic comes from balance and control in chaos. It is reasonable to assume that a naturally more chaotic individual could have an advantage in magic. Perhaps the answer is as simple as: Tissaia sees Yennefer as being the most chaotic student. She teaches Yennefer to control her inner chaos, which leads her to being a good sorceress.
Side note: We also witness Yennefer create a portal on her first attempt, using the Feainnewedd herb. Istredd is clearly shocked at this intuitive control; another hint that Yennefer has some extraordinary abilities.