What is the best supplement insurance for seniors?

The "best" supplement is the one that you can afford. Here's how the supplements work: Medigap, also called a Medicare Supplement, basically pays the portion of medical expenses that Medicare expects you to pay. Medicare is an "80/20" plan, meaning Medicare pays 80% of the bill and you pay 20% of the bill. Medigap can pay the 20% for you. Depending on which Medigap plan you choose, the plan can also pay the (2009) $1068.00 hositalization deductible for you and the $135.00 doctor's office deductible for you.
Now, generally plans A-F are offered by insurance companies. The cost of the plans will vary from company to company BUT the overall coverage of the plan is the same as regulated by Medicare.