I was reminded at the store today that some people still have their original Wii, and haven't upgraded to the WiiU. It made me think that there are some really good games out there that if you didn't play on the Wii, you still should. I would also add that you will have a better time playing these games on your Wii hooked to a good TV using the component video cables than you will playing those games on a WiiU because of what the WiiU's upscaling looks like.
Here's a quick list of my personal favorites for the Wii that you might want to go back and play if you accidentally skipped them. The more mature-rated games are further down the list.
The Legend of Zelda:Twilight Princess (single player)
The Legend of Zelda:Skyward Sword (single player)
Super Mario Galaxy 1 and 2 (mostly single player, but a second player can assist a little)
Super Smash Bros. Brawl (up to 4 players)
New Super Mario Bros. Wii (up to 4 players)
Mario Kart Wii (1-4 players)
Excitebots Trick Racing
Wii Sports Resort (1-4 players)
Most of the LEGO games (Star Wars 1-3, Indiana Jones 1&2, Batman 1&2, Pirates of the Caribbean, Harry Potter 1-4 and 5-7, Lord of the Rings)
de Blob and de Blob 2 (single player - the first game is groundbreaking, but the second game is easier to control because jumping was mapped to a button instead of waving the Wii Remote)
Kirby's Epic Yarn (1-2 players, unbearably cute for some people)
Okami (single player, especially good if you liked the Zelda games )
Sengoku Basara (1-2 players. #1 game at my house by total hours played, but not for everyone)
Resident Evil 4 Wii Edition (single player, Mercenaries mode is still a lot of fun for a long time after you've finished the main story.)
Tatsunoko vs. Capcom (1-2 players, the only conventional fighting game on the Wii.)
No More Heroes 1 & 2 (While its status as 'Mature' content is often questionable as some of the humor is rather juvenile and crude, the games have a unique presentation and narrative without being so artistic as to be a drag on actually playing the game.)
Metroid: Other M (Single player. The game is ridiculous in many aspects, and there's no reason Samus needs to talk, but it was full of interesting combat and exploration and a streamlined control scheme that uses only the Wiimote. Metroid Prime Trilogy is also a great game that used to be three great smaller games, but the controls are more complicated.)
Sin and Punishment (single player, an interesting take on the rail shooter genre. I was glad that I picked it up for $20 when I could as copies have crept back up to $50 in some places.)
Pikmin 1 & 2 New Play Control ( story is 1 player in both games, Pikmin 2 has some multiplayer vs. games available after finishing the main game. They are conversions of a couple of the best Gamecube games that Nintendo made.)
I might also suggest the Rayman games and the Rabbid spinoff games, although they are not in my personal collection. I don't put games like Call of Duty or Tiger Woods or Rock band on this list because if you really wanted to play those games when they came out you're unlikely to be a Wii owner.
Feel free to pick on this list or suggest more.