I would never recommend someone to play with an outdated controller which requires an expensive adapter, over playing Smash with a better and modern controller, which doesn't require an expensive adapter. So if you aren't overly attached to a GC controller, go with the Wii U Pro Controller instead.
I respect your preference but what? GC controller is not even outdated for Smash Bros.
"overly attached"
Geez, dude, generalizing much? Your post comes across as a bit elitist, which is no different than people belittling others for not using the GC controller.
Anyways OP, I find the Gamecube very comfortable for me, especially the wavebird. I feel I'm able to hit the c-stick quick right before having my thumbs going back to my A/B buttons. I'm also a big fan of the button shapes (with the A button which I commonly use being big), so it felt like a very ideal Smash controller for me. Unfortunately the adapter's price inflated due to price gorging jerks on amazon and ebay. Hopefully they'll restock the adapter.
I feel the GamePad is also pretty comfortable, even though I find it a bit slower as a 2nd stick user (due to its position compared to the buttons). But other than that, it's a great controller. A nice way to relax and play it while laying down. It's almost like playing it on a handheld with a very console-like controller!
I have yet to use the Wii U Pro Controller for it since I don't own one, but I'd imagine it probably being more comfortable than the GamePad due to its more traditional shape.
But everyone is different with how they play, so naturally people may find other controllers comfortable and faster/better. So it's best to experiment with other controllers; go with whichever feels most comfortable to you! There's about 6 options: Wiimote (sideways), Wiimote w/ nun chuck, Classic Controller (Pro), Gamecube, Pro Controller and GamePad. There is no wrong answer, just find the best controller for you!