I finished another run in The Last Days of Middle Earth (TLD - a LotR mod available for both original M&B and Warband) over the past weekend, crushing the feeble forces of Gondor and the silly elves (once again) for the glory of the great Khan of Harad and its ally Mordor, then dealing with the traitor Saruman. Still pretty good, even after finishing it several times on each side of the war. Unfortunately, it doesn't include kingdom building; you're always a vassal of someone else.
Gekokujo is also surprisingly good, set in feudal Japan.
Brytenwalda is very professional and polished, but I don't really enjoy the dark British Isles setting and overall game balance that much. Got as far as fielding a very competitive army, but never played to the point of establishing a kingdom.
If you haven't played the Star Wars: Conquest mod yet (ported to Warband from original M&B), it's worth it just to see just how far you can take the M&B engine. Unfortunately, it starts crashing for me about half-way through a campaign, despite memory patches that work for other M&B mods, and are virtually required to run some like TLD or SWC. The revised game (the New Order) for Warband doesn't appeal to me nearly as much for some reason, and also started crashing repeatedly in the mid-game until I gave up on it.
I found the controversial and slightly silly pseudo-historical post-WWI "Red Wars" rather amusing for the first couple of in-game months while you build up your band of retainers and companions. Then I tried to assault a fort. That turned out to be badly broken (a couple of forts don't even exist on the tactical map, so you're fighting in an open field with both armies dumped together on the same edge of the map, while other forts have no normal "door", so you more-or-less have to struggle to filter through a tiny gap in single file while the defenders throw grenades), and the game goes downhill fast from there. It's close to being an excellent mod, but a few serious bugs and unfinished scenes ruin it.
One modder named Gabrilduro started at least half a dozen different somewhat historical mods for various later timeframes (French and Indian Wars, American Civil War, post-ACW western, etc., and they all LOOK very good, but all are incomplete and buggy. It's a pity that he either couldn't find someone to work with in order to finish those projects properly, or never stayed focused long enough to do so himself. As they are, I can't recommend actually playing them, although several may be worth seeing for the nicely done uniforms and equipment.
There was an interesting Napoleonic wars mod for original M&B which included several tricks that really pushed the envelope in terms of faking an economy (purchasing a business, buying rental properties, etc.), some of which were later implemented in other ways in vanilla Warband, but the mod was never ported to Warband.
I never cared for a few of the surprisingly popular mods that attempt to start the game on 11 and then tried to go upward from there, because there's really no place to go from there. The game mechanics pretty well top out, to the point where any further increases make little difference. I recall starting out one game as a pauper turned to poaching, and the mod put me in heavy chain mail with a two-handed axe....like I'm going to go chasing homicidal deer or rabbits while wearing heavy armor, so I can melee them. Another mod raised the speed of horses to about double or more, so you had 60 mile per hour horses. Blink once and you just missed the entire enemy army. Sorry, but the equine turbocharger hadn't yet been invented back then.
Most of the "overhaul" mods have at least one very good or brilliant feature, but another that makes me just stare and say "WHY would you DO this?".