So I bought an Xbox360 during my stay in the US more than a year ago and altho I got to play quite a few games, I haven’t played since and am now trying to play catch up with current gen gaming on that system. I could list the games I played but I’m looking for your opinions on what you guys think are the absolute essential 360 games, so I guess there’s not much of a point.
Thanks in advance. 
(bear in mind i don’t own a Wii or PS3 so cross-platform titles are ok too).
Definitely Child of Eden
Alan Wake (it gets a bit repetitive later in the game though, but overall it’s worth checking out)
Halo 3 (if you played the first two, the game gives the trilogy closure and is overall a solid title)
The following are on PC as well, and some of them (such as Mass Effect) will better. The 360 version of Mass Effect has some framerate issues.
Limbo
Rocket Knight
Mass Effect (the first one is worth playing; I’m about the start the second one)
BioShock
Ninja Gaiden 2 - (I imagine you’ve already played, iirc you were a fan of the first game? just treat it as a completely different game, almost as good in it’s own way, but definitely not as good, be ready for lots and lots of PAIN))
Dead Rising - Still something about this one, nothing else has defined “next-gen” any better for me.
Bayonetta - I can despise so much about it, yet I keep going back for more anyway. Sooooo bloody gorgeous…
Portal and Braid - If you want some serious action puzzling with a glorious personality, either fits the bill. Both on XBLA though for Portal the Orange Box might be a better deal, if less “complete” I guess.
Enchanted Arms - I recently returned to this one, and was able to appreciate it for what it is much better. Perhaps it’s greatest crime is how much it can remind me of PDS, and how awful it then seems in comparison. But it doesn’t actually ask to be taken seriously, and if you can get past that there’s a lot to admire.
Death Smiles - On my mind because I recently got it on impulse, NOT disappointed. It’s a phenomenal 2D bullet hell shooter, just about the right balance of chaos vs forgiveness for my old reflexes. lol
Trials HD (XBLA)- I’m including this more because it’s such a cult, and deserves it’s status. Though it’s personality really disagrees with me past a certain point, or I just plain suck at it.
Toy Soldiers (XBLA)- I dunno, I just really enjoy this game, hard to justify, kinda short and sweet. But so well executed on every level.
Radian Games: Joy Joy (Indies)- Just take a look at it, decide for yourself.
Probably forgetting some that I’d include on a different day. Any Halo game or Gears are fully recommended, I’ve actually had more fun with the new franchise this gen I think. Child of Eden & Rez HD both, absolutely. I still haven’t picked up Alan Wake but been meaning to forever, what I have seen and tried of it is very unique and impressive. And if you didn’t try Too Human already… I would almost say don’t. But who knows, some people seem to have made peace with what it is, I certainly can’t. 
If you like fighting games, I found Super Street Fighter IV to be pretty fantastic. If you like 2D shmups, Deathsmiles is good (and on XBLA Trouble Witches Neo).
Dead Rising is a lot of fun and, even though I haven’t played it yet, the second one should be better.
I was impressed by Sonic & Sega All-Stars Racing for being a really good game in a similar vein to Mario Kart.
Suggestions with the assumption that you do not have a gaming PC (former PC gamer here who just hasn’t spent the money to upgrade his PC. Why when all the games I want are on the 360 and it only cost me $200.)
Deus Ex Human Revolution. (Playing this now and it rocks.)
Halo Reach (I wasn’t a big Halo fan, but this game is solid. Just mute your mic if playing on Live).
Mass Effect 1 and 2 (3rd one on the way, can’t wait.)
Elderscrolls Oblivion (Played it on PC, brother had it on 360. It is worth it if you don’t have a PC.)
Bioshock (1st one is a good play through, 2nd one was boring)
Mirror’s Edge
Sega’s Ultimate Genesis Collection (what a deal!)
Gears of War 1 and 2 (single player is okay with a bud, the online is a blast though)
Left 4 Dead 1 and 2, Orange Box (Better on PC and you don’t need a really fast rig, but if you don’t have an alternative I have tried all these and they look pretty good if you have an HDTV).
Panzer Dragoon Orta is a must with Backward Compatability
Mandatory Disc-based Titles
Mass Effect
Mass Effect 2
Bayonetta
Portal
Portal 2
Fallout 3
Gears of War
Gears of War 2
Dragon Age: Origins
Batman: Arkham Asylum
Child of Eden
Dead Space
Dead Space 2
LA Noire
Tales of Vesperia
Bioshock
Mandatory XBLA Titles
Torchlight
Bastion
Rez HD
Ikaruga
Geometry Wars 2
Shadow Complex
Scott Pilgrim
Puzzle Quest
Mandatory Upcoming Titles
Radiant Silvergun HD
Guardian Heroes HD
Gears of War 3
Mass Effect 3
Project Draco
Batman: Arkham City
Bioshock Infinite
Lollipop Chainsaw
Some of the disc based titles that Parn mentions are on Games on Demand as well, with varying prices. NZ$30 for Bayonetta, for example, isn’t bad. Others tend to be overpriced compared to their physical counterparts.
Mine would be
Ninja Gaiden II
Blue Dragon
Lost Planet
Battlefield Bad Company II
Castlevania LOS
Enslaved
Ninja Blade
GRAW II
FEAR
GOW II
Child Of Eden
HALO III/Reach
Half Life
Singularity
Blur
Tomb Raider Underworld
Alan Wake
Brothers In Arms
PGR 3
COD IV : Modern warfare
RE 5
Rainbow 6 Vegas
The Darkness
I would also second RE5, Fallout 3, Dead Space 1 and 2, and add the arcade game Marathon for some old school FPS action. Every time I compile a list of great games, I always seem to miss some of the good ones I have played myself.
The 360 really does have a solid amount of good games these days.
Thanks guys for the suggestions. I have played quite a few of the ones you mentioned.
I’m surprised everyone liked Gears of War so much. I played it but I didn’t enjoy it very much (and I thought I would - I think the mythos and artwork and story put me off too much…but then again I wasn’t expecting to love an Epic game). Then again I rented most games i played and played them in quick succession (cause where I live you can’t rent games) so by the end I think I was beginning to develop spoiled gamer syndrome. I liked Dead Space and yet I know it didn’t have the same effect on me it could have if I wasn’t on a gaming spree.
Mass Effect 2 was my favorite game, can’t wait for the third installment.
From your lists I think I’ll need to play Portal (everyone raved about it so much) and maybe Oblivion and/or Fallout 3. As for the Live titles…I don’t really have Live.
Child of Eden and Rez HD I’m kinda dying to play because I’m a huge Rez fan.
Final question : is being a fan of previous Halos (even tho I liked 3 slightly less) a good indication on whether or not I’ll like Reach?
On it’s own I think Halo: Reach is easily the better game (than 3), but some people seem to dislike the Spartan team angle. But it at least feels closer to the sandbox gameplay of the original than anything since, though ODST also gets props in a different way. Hard to say, the production is better than ever, but the story also feels a little thin. If you were on Live I’d say it’s absolutely mandatory to try ODST or Reach for the Firefights, but if you have liked replaying the previous Halos I can’t imagine being too disappointed, I wasn’t.