As someone who owns all three, I think Nintendo Land is the best option.
Wii Party U has some fun games (including ridiculously fun tabletop games) but overall it's an incomplete package. Mario Party 10 is a nice diversion with a group of friends but it's not particularly deep or surprising.
Nintendo Land, though, is solid. There are 12 mini-games, almost all of them winners. In terms of gameplay, replay value, and presentation, Nintendo Land is, for me, the best investment.