1.Hotkeys
2.Better Time
3.Party Screen Enhancements
4.Settlement Icons (Unsure which fix version works for current builds so link is to the original mod)
5.Equipment In Encyclopedia
I consider these pretty much equally essential so the order is irrelevant.
1. Being able to control most menus with the keyboard instead of clicking is a huge improvement, since it released it has always been on in my playthroughs.
2. Increasing speed even further with an extra speed option. Must have.
3. Sorting party, easily setting upgrade paths once and then simply auto upgrading, transfer up to limit. The devs have been hinting at some kind of rework/improvement to the party screen, hopefully it implements some parts of this mod.
4. Rather than visiting every village searching for quests / noble units having it show up on the nameplate when in visual range, genius.
5. Some patches ago the devs added equipment to the encyclopedia, before that patch the only way to properly inspect a unit was searching the xmls. I found this very annoying, since no one else was making this mod or something similar, months after release, I finally gave up hoping for someone else to make it and did it myself. Since I had access to the unit and every relevant factor for it I also added extra information such as total armor values, skill effects and the different equipment sets. When the devs changed the unit page they only added the first equipment set (without horse/horse harness for some reason!) to the page. Therefore I still consider my mod essential as it provides all significant info about the troop, not just part of it. (Also in the latest verison it can be used to edit the units in game which should help modders test out their units/changes easier)
As a modder however, by far the best mod for bannerlord is
Better Exception Window. Instead of people reporting a crash without any useful info BEW enables them (usually) to provide the exception that occurred and the call stack. This makes it much easier for the modder to find and fix the issue. I believe this info is already generated by the crash logs but BEW make it so much easier for the users to simply copy/screenshot the relevant parts. It doesn't actually do anything in game however (the modder could/should just debug with a decompiler instead of using BEW), it is only useful when things aren't working properly and it only really helps the modder which is why I would not consider it essential to the user.