My favourites are The Cave and Little Inferno. These games just have the humour and style I love.

My favourite multi player games are definately Chasing Aurora and Trine 2. Nano Assault Neo is a close third though. They all have their qualities, but you simply can't have more fun with a 2P game (Nano), than a 3P game (Trine) or 5P game (Chasing Aurora).
I guess The Cave is a great multiplayer game as well.

I also have Puddle and Mighty Switch Force! and both are good for different reasons, but Puddle has more fresh ideas, online leaderboards, and a cool subtile narrative/plot, three things I really miss in Mighty Switch Force!.

So my ratings are

9/10 The Cave
9/10 Little Inferno
8/10 Chasing Aurora
8/10 Trine 2 - Director's Cut
8/10 Nano Assault Neo
8/10 Puddle
7/10 Mighty Switch Force! - Hyper Drive Edition

I got a Nintendo Points card just to get Balloon Fight for 30 cents but have no idea of what else to get. I hear Trine 2 is really good.

Trine 2 IS really good. The physics can be a bit buggy at times but the art style is amazing.
The game play is pretty simple and not very challenging (playing on normal) however some of the platforming/puzzles will really test your wits.

Best game I own for the Wii U so far (which isn't saying much, I only have 3 games so far).

I'd recommend most of the Indie titles. There isn't a bad one there IMHO. Best ones in my opinion are Zen Pinball 2, Little Inferno, Trine 2, Mighty Switch Force HD

I really hope you didn't get a Nintendo Points card...

Runner 2 is the best game on the eShop atm.

My new ratings:

9.2/10 Toki Tori 2
9.1/10 The Cave
9.0/10 Little Inferno
8.6/10 Zen Pinball 2
8.4/10 Chasing Aurora
8.3/10 Trine 2 - Director's Cut
8.1/10 Nano Assault Neo
8.0/10 Puddle
7.3/10 TANK! TANK! TANK! DL
7.2/10 Mighty Switch Force! - Hyper Drive Edition
6.0/10 Kung Fu Rabbit

New additions:

Toki Tori 2
An original, polished, beautiful openworld 2D puzzle adventure. It's bursting with fresh ideas, lush environments and cute creature design, as well as a great soundtrack. But beware: it's not a platformer - as in 'no jump'. You can't rush through the world as you have to solve puzzles to get forward. It's a game for explorers, collectors, thinkers. Try&error is part of the game concept as well, but there's no punishment, only rewards. My new Wii U eShop favourite for sure.

Zen Pinball 2
I bought 11 tables so far and they're all of superb quality. Even the Marvel ones, a franchise I despise, is fun to play in (zen) pinball form. The game was updated some weeks ago but you still can't sort the tables. While not gamebreaking it's a bit of a mess for people who like to tidy up their collections.

TANK! TANK! TANK! DL
Originally a retail title, the DL version offers you content in DLC packs. It's a simple action game that's quite fun in multiplayer. 9,99 Story mode gets tiring and repetitive as you have to repeat nearly 40 stages almost 20 times to unlock everything. My Kong mode is just an asymetric minigame not that fun either for the price - in fact 1.99 should be its regular price at most, not unreasonable 9.99. Frequent loading times, horrible voice acting and unskippable cutscenes lower the experience as well.

Kung Fu Rabbit
A quick smartphone port and unimaginative Meat Boy clone with cheap graphics but cute art style, and only 2 or 3 abysmally short track loops (no, it's not what I call music or even soundtrack). Took me only 5hrs to fully complete. However, the 160 extremely short and rather easy levels were strangely addicting to beat. Good controls and classic concept together with the low price of 4,99 levitate this above mediocrity.