Start with the drug plan, every year
Part D formularies and premiums change every January, and a plan that was cheapest for your prescriptions last year is frequently not the cheapest this year. Re-shopping takes about fifteen minutes with a full medication list and often saves several hundred dollars.
Check that each drug is on the formulary at the tier you expect, that your pharmacy is in network, and that no prior authorization has been added. The $2,100 cap on covered drug spending protects you from catastrophe, not from a plan that does not cover a drug at all.
