GSMITH,
Your best bet for that car is OEM sized Michelin "Energy MXV4 Plus" tires. They are hands down the best touring tire you can buy. In my experience the right tires are worth getting. When you consider that everything your car does depends on those four little contact patches. Besides, if you consider "how much per mile" difference you pay for good vs cheap tires...

If we are talking a W140 500SEL then your OEM should be 235/65/16. If we are talking W126 then OEM is 205/65/15 or 205/70/14. Since you say you have 15" wheels I would guess your size is 205/65/15. You may want to give the staff at Tirerack a call to get a good second opinion.

Hope this helps...Lee