"Best" is a very subjective word. I'm sure if you asked everyone on this forum, THEIR car would be the "best".

Each version has it's own strengths and weaknesses. The 123's are getting old and parts are harder to come by. The early 126's are in a similar boat, the late 126's (300SDL, 350SD, 350SDL) have a different engine that seems to carry a love/hate reputation (I love my 300SDL for the record). The 124's were broken into a couple of generations, the earlier ones had the same engine/transmission as the 2nd gen 126's, the later generation had the 2.5L. The 1995 had the N/A 606. '96/97 210s had the N/A 606 and the wimpy transmission you mention. The '98/99 was turbocharged and came with the much improved 722.6. Some here love the '05/06 CDI's, but be aware of the SBC issues and the injector "black death" that requires attention. They seem to be pretty solid cars otherwise. The later diesels just don't seem to be as well made. Issues with oil coolers, transmission problems, electronics, emissions controls, etc.

Making a long story short, there is no "best", it's subjective to who asked the question!

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]