Mario Kart 8 -- The biggest influence on my decision to get Mario Kart 8 was probably my daughter's full bloom into a video game fan with the presence of the Wii U in my house. The shared experience has made high-profile multiplayer games much more important to me now and you can hardly get higher profile than this! My memory of playing the original 20 years ago was positive enough to make this game a day-one purchase (and then my partner's kids made it easy to justify getting extra Wii Wheels and matching remotes) and I have no regrets, though my bank balance may say otherwise...
Mario Kart 8 is a blast to play, it looks beautiful and the motion controls are every bit as nice as those in Need for Speed U, though we're in Nintendo-themed lands with colourful characters and vehicles. I've barely played online, but those matches have been okay; mostly it's been a drive to get all the unlockables, with loads of vehicles, characters and flying apparatus. When you're done getting the golden kart and the rest there's some nice (and reasonably-priced) additional content to buy in the form of a couple of track packs (with hopefully more to come). I just can't imagine not having this game if you have a Wii U unless you really hate racing games - join the fun!
Kirby's Adventure (VC:NES) -- Nintendo's early gaming bread-and-butter was platforming games with Kirby adding the twist that the main character can inhale enemies and fire them at other enemies or use them like power-ups to gain an ability.
I haven't finished it, but I can't say I find it terribly challenging: Kirby's floaty jump ability and lack of tricky jumps means that enemies are your main hazard and inhaling them makes them easy to neutralise. There's a quasi non-linear level progression where you often have more than one choice of level to complete and periodic boss battles punctuate the "find the exit door" level progression.
It's okay, but didn't light my fire enough to keep me engaged. Worth a look if you're curious about Nintendo's b-list titles, but otherwise the Mario games are more accessible and tough to beat.
Super Dodge Ball (VC:NES) -- I generally dislike sports games, but I enjoyed the arcade version of this title. The NES version isn't as good as the PC Engine one, graphically, but it is a lot easier which is its downfall: on the default difficulty I beat all comers in less than an hour. Given the PC Engine version was never released outside of Japan it's likely this is the only way you'll ever play it.
Wario Ware Inc. (VC:GBA) -- One of the first Gameboy Advance titles to gace the Wii U Virtual Console and one of the best. If you liked Game & Wario then this is a no-brainer. Loads of micro-games that can be played in seconds; often with a single button press. There's more to unlock as replay incentive, with additional play modes and extra difficult versions to tackle - if you can rise to the challenge!
Legend of Mystical Ninja (VC:SNES) -- A combination of side-scrolling brawler and RPG. Although the RPG bit differentiates this game from the typical beat-em-up like Final Fight, it also makes it less pick-up-and-play due to it not being always obvious which way to go to progress if you put the game down for awhile. Given the lack of inspiration from repeated enemies throughout, that's likely to be a problem. I could have passed on this without feeling I'd missed out on anything, but if you're looking for something different from the SNES catalogue, you could do worse.