Well, what STEM major are you? Assuming you are in chemistry (given you are posting here) I can tell you the order I took my sciences... Assume they are two semester courses unless I say otherwise
Freshman: General Biology, General Chemistry, Calculus
Sophomore: Organic Chemistry, General Physics, Inorganic Chemistry (second semester)
Junior Year: Physical Chemistry, Organic Chemistry 3 (first semester), Instrumental Analysis (second semester)
Senior Year: Biochemistry and Advanced Analytical Chemistry (first semester)
This worked decently well for me. I would say the hardest year for ME was sophomore yr because organic, despite doing well in it, was a very time consuming course that left me with no time to really keep on top of my physics.
Junior year was very challenging because of the P-Chem series but I had my highest GPA that year so I think I just got used to it after sophomore year, since that is a major adjustment.
Hopefully this helps a little. Providing more information on what major you are (STEM is very broad) would get you more helpful advice.