I use the Gamecube controller, and before I got my adapter, I used the Hori Battle Pad (It's a classic controller, but it looks and feels like a Gamecube controller). I haven't tried the Pro Controller (it's so freaking expensive), but I hear it's good and I'll probably buy it when I have money.
I've actually enjoyed using the Hori Battle Pad.
I struggled getting used to playing Smash Wii U with the Game Pad and regular classic controller and never really got the hang of them. After playing a bunch of Smash 3DS, I was okay with using the 3DS controller, but I was a little worried about wearing out the circle pad and you can't use it as a controller in all game modes anyway.
Given the Gamecube controller adapter's scarcity, I opted for a Hori Battle Pad. Now that first party adapters are a bit more readily available, I've considered just switching to the proper Gamecube controller, but I haven't really felt much of need to do that. I don't have a bunch of people to play with locally who would want Gamecube controllers and the Battle Pad is working out fine for me. Plus, unlike the Gamecube controller adapter, the Hori Battle Pad works in other games, including Mario Kart 8 and Brawl when played in Wii Mode, so I've been able to get some extra mileage out of it.