I guess I would choose "Game & Wario," because I never owned a Wii U.
I just now remembered that WarioWare Mega Party Games is a remake of the original with multiplayer. Also in a way, WarioWare Gold is a like a remake/mash up of multiple games.
Eh, we already got Gold, which is effectively a remake/soft reboot of sorts of the previous games (okay, the story is new, but still), so I feel that quota has already been met.
That said, If I'm not allowed to cheat and I have to pick one, then probably Mega Microgame$, as it is the most aged of the series. Polish the microgames up to modern standards, add the content from Mega Party Game$. Maybe even go the extra mile and include the later characters and some new minigames.