My answer would be an 8 bale in-line self dumping round bale trailer. I bought it in 2011. Before that, I had a 20 foot implement trailer that I could haul 5 bales with. It required that I have a tractor on both ends in order to load and unload.
Now, I need a tractor to load with but I don't need anything to unload. With a 40 minute round trip to my hayfield, hauling 8 bales instead of 5 is HUGE. Must quicker to load and unload. BIG improvement... Can't believe I did without it for so many years.
Buying my first John Deere Cab tractor would be my next vote. And while that increased my comfort, the bale trailer increased my productivity. So the bale trailer gets my first vote.