Short answer: The best time is your six-month Medigap Open Enrollment Period, which begins the first month you are 65 or older and enrolled in Part B. During that window an insurer cannot deny you or charge more based on your health; afterward, it usually can.
Federal law gives everyone who is 65 or older and enrolled in Part B a one-time, six-month window when Medigap coverage is guaranteed issue. In those months you can buy any standardized Medigap plan sold in your state even if you have health problems, and the company cannot deny coverage, raise the price because of your medical history, or apply most pre-existing-condition waiting periods.
Once the six months pass, an insurer generally does not have to sell you a policy and may charge more or decline you based on health, unless a guaranteed-issue right (also called a Medigap protection) applies, such as losing other coverage. People under 65 who have Medicare because of a disability may not get this right in every state. Because every plan of the same letter covers identical benefits, it pays to compare prices across companies before you buy.