First off, you can't play GCN games directly on the Wii U (you can on BC Wii units, but not on the Wii U), so that makes the GCN controllers only useful for Smash at the moment (though more use could be potentially planned).
Second, the bare minimum you should have for MP on Wii U would be four Wii remotes (preferably motion plus types, but not many MP games require motion plus) and four nunchucks, relatively inexpensive and will cover most all games for MP comfortably.
Getting one (or two) Classic Controller (or Classic Controller Pro) will cover your needs for playing most all other games via Wii mode, and the CC and CCP will work on a variety of Wii U games, so overall this one is very universal.
Finally, I would get at least one Wii U pro controller, but hold off on getting more at the moment (since they are pricey). One will help with playing MP games on Wii U with one other person most comfortably, but in many cases Remote + Nunchuck / CC / CCP will work just as fine for 3-5 player games.
- Vinny likes this