Best S Class
Well, for my money, a 300SE or SEL is the best choice. Model year is probably less important, although '89-91 had alot of refinements.
I have a 90 300SE and it's a good ballance of performance, reliability and economy. The 124 engine was the workhorse of the MB line during that period and except for a few quirks, has proven to be very reliable. It may be one reason diesels became less popular.
Some might say it's underpowered, but, I came put of a 300SD, so to me it's a rocket. You should see fuel mileage around 20 MPG and they require less maintenance than a V8.
In any event, no matter what you choose, you'll be driving one of the best cars ever built...by anyone. They are a to drive and own.
Best of luck!
91 300E 120K
90 300SE 275K (sold)
92 BMW 525iM 120K
90 BMW 525iA 175K
85 300D 175K (sold)
84 300SD 245K (sold)