I only have Wii Party U, but they said a lot of great things about Nintendo Land. and Mario Party 10 due to 'Mario' it would sell good numbers. Nintendo also has Game and Wario as well.

Can Nintendo really merge or combine all these games into 1? I know they'll sell more with multiple IPs, but for the fans, merging these games into 1 would do great. but across a console lifespan it might not be best for business. I just hope Nintendo could do more on other genres than party games.

Depends on what you want them for.
If it's for mostly multiplayer then NintendoLand probably isn't the best option as it only has a few multiplayer games. Granted, I absolutely love the multiplayer experiences it does have to offer.

I tend to enjoy the minigames in Wii Party U a bit more than Mario Party 10 as well. As someone above mentioned, the tabletop games it has are quite fun, as is the Pictionary mode it has.

All 3 are solid choices, but it really depends on personal preference. I don't think you'd be disappointed with either if you like those sort of games :)

I love Mario Party 10, all of my friends that I bring over to play it loves it.

Wii Party U is great fun in a group.

I haven't played Mario Party or Wii Party, but Nintendo Land has some really excellent games. Even this far out from launch it's provided some of the Wii U's best experiences in my opinion. Metroid Blast in particular is fantastic multiplayer (if you've got Wii Remote Pluses) and DK Crash Course is straight-up HARDCORE.

Easily Nintendo Land!  Wii Party and MP10 are meh at best if you ask me.