I put Michelin Cross terrains on both a 2007 GL450 and my wifes 2004 ML500.
Great tire for a wide range of conditions, and did very well in the mid atlantic blizzard of 2009.

I'm now needing tie for my wife's 2008 ML550, and the best option I can find is the

http://www.tirerack.com/tires/tires.jsp?tireMake=Continental&tireModel=ExtremeContact+DWS&partnum=55WR9ECDWSXL&vehicleSearch=false&fromCompare1=yes

A smaller wheel size would be better & offer more rubber options, but I'm not incline to spend the $$ to get whole new set of rims