Well, what are they.
 
Mine are-  
1. Shadow Complex- for its MetroidVania style and fun gameplay.
2. Secret Of Monkey Island: SE- Because... its got a three headed monkey of course.
3. Castle Crashers- Not so fun by yourself but loads of fun with friends.
4. Marvel Vs. Capcom- Fast and easy combat and a lot of fun with friends... not so much online with all of the damn Cable spammers and shit. 
5. Castlevania: SOTN- One of the best PSone games ever. One of the best XBLA games ever.
 
Honorable Mentions- 
PEGGLE: How could I forget you. So addicting.
Blacklight: Tango Down- though its new, its a lot of fun. Its like MW2, but I'm always good, better customization, no annoyingly cheap killstreaks and no glitches and Hackers. MW2... What happened. 
BattleField 1943- Really good, but with only three maps and three classes, it can get quite old. 
Trials- Loads of content but can get frustratingly hard. Still fun though.
 
New XBLA games that may be on this list in the future-
Castlevania: Harmony of Despair- Its SOTN style with multiplayer.
Limbo- Watch the Quick Look if you haven't already watched it 80 times while drooling. It looks masterful.

SOTN
Shadow Complex
Monkey Island 2 Remake (so much better than the first remake)

"Monkey Island 2 Remake (so much better than the first remake) "

Top 5 would probably be:
 
1. N+ - By far, adore its simple platforming.
2. Ikaruga - Still as good as it was on the GC.
3. Castle Crashers - Loads of beat 'em up fun with friends.
4. 'Splosion Man - Another brilliant platformer.
5. Perfect Dark - Retains all the jank of the original, how a remake should be.

Puzzle Quest and Castlevania: SOTN are my favourites. Marvel Vs. Capcom 2 as well although I have it on PS3. I also really like Shadow Complex and 'Sposion Man and can't wait for Limbo to be released.

Trials HD and Castle Crashers are the only two I would buy if I had no XBL arcade games.

Shadow Complex
Peggle
Trials HD
N+
 
Tons of other great games I've played. But these are just on a higher level for me.

1.  Shadow Complex 
2.  Trials HD 
3.  Castle Crasher (but only if you have a couple of friends) 
4.  Perfect Dark 
5.  The Banjo Games (brings back such good memories)

1. Shadow Complex (God, I loved that game. I am pretty high on the melee kill leaderboard as well) 
2. Castle Crashers (Got that weird glitch that makes you kill everything in one hit and be immortal, so it's kinda dull now)  
3. Battlefield 1943 (I always like a good Battlefield game) 
4. MVC2 (Took me awhile to get used too, but now I enjoy it) 
5. Pac Man CE (Gotta end with a classic)

Shadow Complex, Geometry Wars, Chime.

Banjo Kazooie!

1. Castlevania- I've beaten it 2 times, every time to the 200.6%
2. Shadow Complex- I've beaten it too many times. Seriously, I'm sick of this game, but I love it so much..
3. DOOM 2- Just cause I friggin love doom 2.
4. Geometry Wars RE2- Awesome arcade game, still trying to earn that Smile achievement.
5. Secret Of Monkey Island Remakes- I love the humor, the riddles frustrate me sometimes. Eh, whatcha gonna do.

Mine's definitely Portal: Still Alive. 

HOW HAS NO ONE MENTIONED BRAID?

" HOW HAS NO ONE MENTIONED BRAID? "

1. Shadow Complex
2. 'Splosion Man
3. Castle Crashers
4. Battlefield 1943
   
5. Mega Man 9 or 10

I don't get the love for N+. I only played the demo but it feels like a flash game and I can play those for free on the Internet.

I've had the most fun with: 
 
Bionic Commando: Rearmed 
Braid

" I don't get the love for N+. I only played the demo but it feels like a flash game and I can play those for free on the Internet. "

Bionic Commando: Rearmed
Battlefield 1943
Shadow Complex
Dig Dug
Banjo Kazooie

I think Rez HD definitely deserves a mention.

1. Splosion Man
   
2. Geometry Wars 1 & 2
3. Braid
   
4. Pac Man CE
   
5. Uno Mother Fucka
   

" HOW HAS NO ONE MENTIONED BRAID? "

Braid was my favorite.  
 
I can't wait for Limbo.

Shadow Complex
Braid
1943
Castle Crashers

The only XBLA games I've played were Braid and Dash of Destruction.  I don't think I need to say which one was better. 
  
I do plan on getting Limbo once that's out.

risk faction
battlefield 1943

Braid is really all you need. However, I do enjoy the Banjo games.

Age of Booty 
Garou Mark of the Wolves

In no particular order:
1. Worms
2. The Dishwasher: Dead Samurai
3. Castle Crashers
4. Zeno Clash
5. Marvel vs. Capcom 2
 
I'm also looking forward to Limbo, Monday Night Combat and the Diablo style Tomb Raider game.

Puzzle Quest 1 and 2 
Shadow Complex 
Castle Crashers 
Castlevania SOTN 
Geometry Wars 1 and 2
Magic the Gathering

Shadow Complex
Bionic Commando Rearmed
Castle Crashers
Alien Hominid HD
'Splosion Man
Puzzle Quest 1 (I have it on PC, I don't imagine it's much different)
Edit: Obligatory Braid love goes here

Trials HD

Magic Sword (the game that comes with Final Fight)

1.  Magic The Gathering 
2.  Marvel vs. Capcom 2 
3.  Perfect Dark 
4.  Risk: Factions 
5.  Castle Crashers 
 
Some games like Virtual on should be on this list if they were a little cheaper.  If Perfect Dark can come out at 800Microsoft Points then why can't other classic games.  Although I will be glad to pay 1200 points when Hydro Thunder comes out this summer.

1. Castlevania: Symphony of the Night -- Still the best side-scroller ever made.
2. Braid -- Looks beautiful, sounds beautiful, and has some of the most mind-bendingly clever puzzles I've ever seen.
3. Shadow Complex -- Classic metroidvania + modern shooter = success.
4. Castle Crashers -- It's silly, it's funny, and it has co-op; what more does a side-scrolling beat 'em up need?
5. Metal Slug 3 -- See #4, and replace "beat 'em up" with "shoot 'em up".

No love for Rocket Riot? :(

Honorable mention - 
Tenage Mutant Ninja Turtles Arcade Game (can go wrong with $5)
Trials HD ( I want place higher if it didn't frustrate me so much on harder levels)
Baby Maker Extreme (I know its a indie game but its so funny)

5. Marvel vs Capcom 2
4. Super Street Fighter 2
3. Shadow Complex 
2. Geometry Wars 2
1. Splosion Man!!!

...Marbe...
...Blast...
...Ultra...
 
...That is all.

My Top 5 -
1.  Death Tank 
2. Trials  
3. Geomery Wars 2
4. Rez 
5. Splosion Man  
  
I've played Death Tank, Trials, and Geometry Wars 2 more than any full retail games. DT and GW2 are great for multiplayer, Trials is impossible to stop once you start.
 
I too am looking forward to Limbo and still need to pick up Braid.

" HOW HAS NO ONE MENTIONED BRAID? "

1. Lumines Live

2. Defense Grid

3. Trials HD

4. Secret of Monkey Island

5. Peggle

1) Bionic Commando Rearmed - An excellent sidescrolling action game with a top notch soundtrack
2) Marble Blast Ultra -  The multiplayer is crazy whaaaaaat
3) Carcassonne - Fun and simple board gaming 
4) N+ - My only proud S-Rank, this game is invitingly tough and a really unique and strong feeling platformer
5) Shadow Complex - The action is remarkably solid and there are a lot of great moments, but I think some of the achievements pushed this too far into a grind mode toward the end