Here is the list of Mathematics Indexed journals on Scopus. The first ten have the highest citescore, meaning the highest amount of citations in the field.
Standards can be quite subjective. Impact factors may not always be what they are purported to be. Beauty is on the eyes of the beholder. A good research should be simply followed, logical and replicable. Journals have their different "qualities", focuses and scopes. No journal is globally the best in terms of quality, focus and scope.
This is an important question. In addition to the helpful answers already given, there is a bit more to add.
In addition to considering Scopus, there is another very important aspect to be considered by aspiring mathematicians in selecting a journal, namely, whether reviews of published articles from a mathematics journal appear either in
zbMATH:
https://zbmath.org/
MathSciNet:
https://mathscinet.ams.org/mathscinet/
The appearance of reviews in either of these journals provides a strong indication that senior mathematicians are both interested in the published results and following the publications by a particular author. This is very important, since it gives each author an indication of the reach of the ideas in a published paper.
I think the index of the journal is the most important factor to identify a valuable journal to publish your work. Then you can consider its impact factor.
I think the index of the journal is the most important factor to identify a valuable journal to publish your work. Then you can consider its impact factor.