"What would you say is the one thing in your bee yard that you can't live without?"
My smoker would be at the top of my list, next jacket and veil, I usually keep a knife with me so can use that if I don't have a hive tool, a frame rest really comes in handy, especially when pulling individual honey frames for harvesting, which leads to extra woodenware to hold frames, which leads to extra hives complete with top cover and bottom board for catching or housing swarms.
Still, there's nothing like having your truck with all your tools handy in case the unexpected occurs such as having to rebuild a box or fix frames, etc...
Obviously all this makes no sense whatsoever if you don't have good access to your hives!
...JP