[Penyngton]

I believe that the 'official answer' is that SageMath is the best of the free software CASs. I personally use Maxima, because it's good enough for my use cases (which involve a lot of finite sets and not many inequalities) and I've found it simpler to understand.

Obviously, Mathematica is the leader in academia generally, as far as I can tell, but I personally haven't had good experiences with it myself, but I also have a strong preference for free software, so I haven't tried to hard to get used to it.