Single best mod for me at present is:
"Character Reload Fix" which allows you to completely edit (i.e. overhaul) ANY character in the game - stats/appearance/gear (new feature) - so you NOT have to be stuck with companions who have traits not suitable to your character, or a giant/dwarf of a spouse, or clan members with hideous dress sense -> this is the GOAT for me.
Improved Garrisons is fantastic in making you able to have your garrison have a function - send patrols to guard your settlements, send reinforcements from an established fief to a newly taken one, etc. Great and immersive, great interface, being able to customize what your garrison does, how it recruits, etc. is great.
Party AI Overhaul is another excellent mod that gives you control over what your (otherwise stupid) clan party commanders do - so you can send them on patrols to safeguard a region, have them follow you (insta-army, even as mercenary), and even to the fine detail of guiding their recruitment policies.
Improved Combat AI - I think this does have an effect on how the (otherwise stupid) troops behave. Certainly have seen NPCs block better in tournaments, and cavalry seem to understand which way the pointy end of their lance should go. An improvement over vanilla, without groundbreaking changes (like some of the other "realistic" mods that cause you to get knocked down endlessly without reason)
The "
True" suite of mods - changes how relation developes, gives NPCs their own opinions, let your companions/clan/spouse develop relation with you over time. Great addition as there are things the vanilla game has in that never shows when you have low relation (like your spouse changing what they say to you at high relation)
Surrender Tweaks - no longer does every NPC party leader have a death wish. When they are heavily outnumbered they may offer a bribe to be left alone. Really nice change from idiot having their troops slaughtered needlessly.
Realistic Weather - it now may rain/be foggy/ etc. in Calradia. Cool mod
Land of Sika - very promising looking total overhaul quite similar in feel to PoP from Warband. Very cool features like being able to build your own settlement