I only own 17 games, so I can't give a top 20. I'm also going to leave out Wii Sports, Wii Play, and Wii Fit because I have a hard time ranking them as games. I did enjoy all three of them though. This is tough, so the games in the middle could probably go in a number of different orders.

14 - Guitar Hero: Aerosmith
13 - Guitar Hero III: Legends of Rock
12 - Zack & Wiki: Quest for Barbaros' Treasure
11 - Boom Blox
10 - Metal Slug Anthology
9 - Excite Truck
8 - de Blob
7 - Okami
6 - Resident Evil 4: Wii Edition
5 - The Legend of Zelda: Twilight Princess
4 - Super Smash Bros. Brawl
3 - Super Mario Galaxy
2 - Mario Kart Wii
1 - Metroid Prime 3: Corruption

Here is my top 20.

1. Super Mario Galaxy
2. Wii Fit
3. House of the Dead: Overkill
4. MadWorld
5. No More Heroes
6. Metroid 3
7. Super Smash Bros. Brawl
8. Mario Kart Wii
9. Wii Sports
10. Zelda: Twilight Princess
11. Wario Ware: Smooth Moves
12. Super Paper Mario
13. Link's Crossbow Training
14. Boom Blox
15. Tales of Symphonia 2
16. Wii Play
17. Tatsunoko vs. Capcom

Those are the games in my collection that I've actually started.

1. Super Mario Galaxy
2. Super Smash Bros Brawl
3. Metroid Prime 3
4. Twilight Princess
5. World of Goo
6. Fire Emblem Radiant Dawn
7. Resident Evil 4
8. NPC Pikmin 2
9. Wii Sports
10. Boom Blox Bash Party
11. Mario Kart Wii
12. Little King's Story
13. Punch out!!
14. Wii Play (tanks was amazing)
15. Super Paper Mario
16. Okami
17. Wii Fit
18. House of the Dead: Overkill
19. The Conduit
20. Mad World

Something like that... roughly (I don't own all of those, so I'm winging it a bit based on impressions)

Faxanadu said: 20? Holy lord, thats plenty. I dont own that many but I have to say that I liked Wii Play more than Zack & Wiki. Shocking, isnt it?

No, I don't think it's shocking.  Tanks is awesome and the end of Zack & Wiki is disappointing.

Not really sure about the exact order, but my Top 5 is crystal clear...

20) Wii Fit
19) Pangya Golf
18) Excite Truck
17) Disaster Day of Crisis
16) Boom Blox
15) No More Heroes
14) Wario Land Shake Dimension
13) Super Smash Bros Brawl
12) Zack and Wiki
11) Wii Sports
10) Mario Kart Wii
9) Resident Evil 4
8) De Blob
7) Super Paper Mario
6) Fire Emblem Radiant Dawn
5) Okami
4) Legend of Zelda Twilight Princess
3) Little King Story
2) Metroid Prime 3
1) Super Mario Galaxy

Plus a special mention for the 4 Wiiware games i've downloaded: LostWinds, World of Goo, NyxQuest, Swords and Soldiers... nearly the same quality/price as the ones above, just downloadable and less expensive... :)

Oh yeah, I'll add that FFCC: MLAAK would be #1 if you could include WW.

I still haven't played all my games so, in alphabetical order

1. Castlevania Judgment
2. Dragon Ball Tenkaichi 3
3. Elebits
4. Madworld
5. Mario Kart
6. Mercury Meltdown Revolution
7. Metroid Prime 3
8. MLB Power Pros
9. Naruto Clash of the Ninja
10. PES 2009
11. Punch Out!
12. Resident Evil 4
13. RE Umbrella Chronicles
14. Super Mario Galaxy
15. Super Smash Bros Brawl
16. The Conduit
17. Zelda Twilight Princess
18. Wii Sports

Adding WiiWare:

19. Bonsai Barber
20. Megaman 9

Twenty Games that's a lot. Some of the games near the bottom I didn't really like, but just added games I've played to fill out the list.

20. MySims Kingdom
19. Call Of Duty Modern Warfare
18. De Blob
17. Mario Party 8
16. EA Playground
15. Carnival Games
14. Wii Fit
13. Super Paper Mario
12. House of the Dead Overkill
11. Boom Blox
10. The Sims 2: Castaway
9. Wii Play
8. Metroid Prime 3 Corruption
7. Boom Blox Bash Party
6. Super Smash Bros. Brawl
5. Grand Slam Tennis
4. The Legend of Zelda: Twilight Princess
3. Wii Sports
2. Mario Kart Wii
1. Super Mario Galaxy

Mario galaxy
Mario kart wii
lost in blue shipwrecked
Zelda tp
call of duty 3
tomb raider underworld
red steel
no more heroes
house of the dead overkill
resident evil UC
Zack & wiki
paper Mario
brawl
sonic sr
tomb raider anniversary
wii music
excite truck
wii sports
BWii
okami

1. Metroid Prime 3
2. Super Mario Galaxy
3. Legend of Zelda Twilight Princess
4. No More Heroes
5. Zack & Wiki
6. Super Smash Brothers Brawl
7. Fire Emblem Radiant Dawn
8. Mario Kart Wii
9. Tenchu Stealth Assassins
10. Mad World
11. Red Steel
12. Wii Sports
13. Soul Calibur Legends

These are the only ones I own. I haven't played my copy of Okami yet.