if you want good story then mh4u. But if you want to hunt lots of monster then XX (or Generations Ultimate)

and in 4u u can play online, while in xx servers are still in working

Go with 4U for multiplayer or if you're new and XX if you're looking for singleplayer and tons of content but no story. 3U imo is the most polished singleplayer experience but doesn't have anything crazy content-wize.