The two mods I mostly play are SS and BC which are both very fine mods which add a lot to the gameplay and are without most of the bugs which plague vanilla. BC is quite a radical departure I think, with its emphasis on the Islamic world with the Christian nations on the fringes being an inversion of the situation in vanilla. The types of armies you will be assembling and facing will be very different in makeup to those in vanilla, especially the further east you go. SS is closer to vanilla in terms of setting, but it has quite a different feel to vanilla, I would say it is actually closer to the original MTW with its grittier, more balanced battles, and in the latest version the campaign map is enormous, so winning a long campaign will take a good deal longer than in vanilla (whilst not making expansion so hard as to suck all the fun out of campaigning).

As pevergreen points out though, they are both huge downloads. Also, if you go for one of these mods, I strongly recommend you also install the latest patches before playing (I think 6.1 for SS and 1.1 for BC), since there are a couple of quite serious bugs in the unpatched versions.

If you want something a bit closer to vanilla LTC is a good choice, but I would also like to add FactionHeir's VanillaMod to the list of mods you should consider, especially if you don't want too drastic a departure from vanilla; VanillaMod basically fixes all the annoying bugs in vanilla and improves the AI but keeps the essential content and feel of the game the same.

Another one to consider is RetroFit, which is basically the vanilla Grand Campaign redone in the Kingdoms engine, with the extra features and partial bugfixing that come with Kingdoms.

TBH, you can't go wrong with any of the mods I have suggested above, they are all very fine. It's more a question of exactly how close you want to keep the game to vanilla.