Are you getting the system at launch? If so, imo NSBM is one of the two "no brainers" to get if there is one. It's Mario, it's a first-party title so you know it'll utilize the gamepad well, and...it's fucking Mario in HD.
It's Mario and it's HD are enough reasons alone. However, you might be disappointed in the gamepad usage. It is really only for a secondary player to create platforms and stun enemies. That can be fun (I played that role at PAX) but I'm not sure it is even used if you are playing single-player. If so, they certainly didn't emphasize that in their demos. It was 4 people on Wii remotes and 1 on the gamepad with much chaos ensuing. But perhaps for single player you can use the gamepad analog sticks to control Mario too (and create platforms?). I also hope it allows for Pro Controller usage. I just absolutely detest holding Wii remotes sideways and using the D-pad to control a game.
The best use of the gamepad will surely be NintendoLand since that was designed with the pad in mind.
Never having played the other games anyone think Scribblenauts Unlimited will be worth a buy? With the coupon at newegg it's only 39.99
Probably. Of all the other games (besides NSMB U) it is the closest to a "sure bet" quality game. It just depends on if you like the concept or not. I haven't jumped on that yet because I still haven't even played Super Scribblenauts on the DS yet, though I did play the original. From what I gather they have refined the formula for this Wii U release so I'm sure it will be a good game. The strange thing about this game is that you can play it pretty much just on the pad without looking at the TV - so that might be a nice game to have for when the TV is being used for other stuff.