RE: The one item you would not go without...
My vote for the "too often ignored or "left at home" item would be a cheap simple GPS ($100-150) and the computer hook up cable to a computer with Topo! software for Montana installed on it (leave the notebook computer in the truck at the trail head). Use the software to make your own custom map showing pre-designated waypoints of landmarks, trailheads, water, saddles, etc. BEFORE you ever go.
IMHO, no single item will give you more confidence and help you master your "never been here before" area better than GPS shy of a scouting trip and maybe not even then.
HOWEVER, get the TOPO! software for your home state and thoroughly master it's use in a local situation first on the highway and then in the filed BEFORE leaving -- this is critical.
Most the successful elk hunters I know navigate to their stands both ways in the dark and when you are a newbie in the area that is just asking for trouble unless you can GPS it. If the older experienced guys don't use this and you are all going to an area none of you have ever been to, AND you really know how to use it and you have it preprogrammed with relevant waypoints in addition to the ones you add as you go, and you have printed out maps for everyone in your team, then you will have something that will be a real personal contribution to your group and you'll feel much more like a player than a tag-a-long.
My $.02, choose what you please.
EKM