Farthest to the right: in the rightmost lane of the highway. (adjective)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "rightmost" in a sentence
"There is a railroad bed that runs across the face of the mountain and just to the left and above the rightmost ship was once the largest stamp mill in the World."
"If there was going to be anyone home, it would probably be in the rightmost room."
"Cases in which it's more objectively useful; I have the mobile versions of a few sites set up as Fluid apps (same thing as Prism, basically); when they open, they automatically sit in the rightmost 28% of my screen which I use for Tweetie, chat windows and these "sidebar" apps."