I hate this question because it is asked so many times. There is no "best weapon" all of them are good in their own ways. It all depends on what you like best and what you are most effective as. I personally mained the insect glaive in MH4U, and did very well with it. Had a very good time with it and loved. I also thought that i was fairly talented ans skilled at the game, but I've played online with some people who were also insect glaives and they were terrible. Not because they were bad players, but just because they didn't work well together with the insect glaive's style. They were great great swords, or double blade users, but they just didn't work out with the insect glaive.
The only way to ave an objective "best weapon" is to choose a specific category, such as raw damage, mobility, elemental damage, or adaptability, or to choose a single weapon type, and find one of the best ones in that single weapon type. Even then you won't be able to get a completely objective answer some of the times. It's just impossible.