Original Wii (2006)

What are the best side scrollers,
to play on this 15 year old console?

My all-time favorite, is "New Super Mario pt 1".
and I would like to try, Luigi's Haunted Mansion
Also interested in, Donkey Kong-world for (Original) Wii.
.. even though, they are not a actual side scrollers.

Did not enjoy, Mario Galaxy or the 1st Paper Mario.

2nd question:
What store bought controllers, can I buy? thinking
8bitDo.. but they are only WiiU and later.

Maybe Kirby's Epic Yarn or Rayman Origins?

Thank you, I guess I am in uncharted enviornment (Nintendo).
I really enjoy, Spelunky and Mark of the Ninja (not intentional, just my fav genre).

And New Super Mario, is very cool, and still challenging to play.

I will check out the games, that you mentioned.

There's also Kirby's Dream Collection: Special Edition, and Kirby's Return to Dream Land.
Now if you want to go into GameCube games (Most Wii has Gamecube support along with 4 controller slot and memory card slot) you could also check out Veiwitful Joe series

If you have SNES or NES Classic Mini you can use those controllers with the Wii (plugs into the wiimote) for games that support classic controllers