I guess you should first properly separate the two W140 categories: pre-facelift and post-facelift.
Pre-facelift run from 1991 to 1994.
Post-facelift run from 1995 to 1999.
Pre-facelift the definite best year is 1994 (late).
Post-facelift models are usually less prone to problems than pre-facelift models, although as said above, the better years are 1998-99.
Try reading this post to learn a little about W140s reliability and pre and post-facelift differences:
http://www.peachparts.com/shopforum/showthread.php?t=19772