Just under the door handle is the best place. I attached mine low enough so that a regular soda can, snapple bottle, or regular size drink wouldn't interfere with the door operation. Of course, if I buy a Big Gulp at the 7-11, I have to remove the drink to open the door. The reason why I didn't mount it lower is that it hits my knee.
I started a post on this cup holder a couple of months ago and it got well over 500 hits and pages of posts. Pictures are posted here showing the mounting location.
Do a search on cupholders, or even do a search under my name 'suginami' in the members section by doing an advanced search and then scrolling through all of my posts.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".