It depends a bit on what you want. If you like the medieval era, those three mods are the ones you should try out. Personally, I played WesW's MedMod a lot. WesW set out to make the game more challenging and the A.I. more competitive and he certainly succeeded in that. The MedMod v2 version doesn't really change the overall game, but makes various adjustments that improve A.I. performance. For the full mod, you have to install MedMod v3: this is a total conversion, and would be the most challenging M:TW mod if it wasn't for the MedMod v4 beta. Sadly, the beta won't be finished, but I am told it is very playable.

There are also modifications that change the game to another era. Have a look in the Engineer's Guild and hosted mods sections. I have heard good things about Hellenic:TW, Napoleonic: TW and Samurai wars.