Most treasured achievement?

[Broady Brio]

Either Professional Mode on Hitman: Blood Money
(Except one level, all done with Slient Asassin rank)

OR

Blue Collar on Saints Row 2
(I don't if it was just me with this but Tow Truck was almost impossible the truck being blown up on the last level of that activity really made me go on rampages then load up again)

 

[KaiRai]

Medal of Valor

You have to beat Mass Effect on Insanity difficulty. It was particularly satisfying since I started a new game instead of using one of my high level characters like I should have. Looking back I also unentintionally made it as difficult as possible.

Also my favourite achievement
that and the stupidly high note hit one on Guitar Hero. Took me long enough.

 

[Flour]

0G - Tail Between Your Legs

Just because it was the most unexpected achievement I have ever seen.(decline a Guitar Hero 3 boss battle)

 

[Cpt_Oblivious]

Using the Search Button

Wow... That's... Amazingly twatty, even for you.

Which bit? Because I've posted that same answer on the other 3 "favourite achievement" threads this month.

 

[Fat Hippo]

Whatever the achievement is for beating CoD 4 in the highest difficulty. That game is pretty damn tough.

 

[Ka_huna]

Yatta!

 

[Samah]

Aye, [Little Rocket Man] from HL2 Episode 2. Every time I see a garden gnome now, I cringe and have nightmares.
For WoW, I would have said [Lil' Game Hunter] for collecting 75 vanity pets, but that's all too common now. I currently have 93 pets, so I'm hoping they make a 100 achievement.

Edit: I didn't notice the "360" part in the OP, but the first achievement is still valid.

 

[CaptainCalvin]

Little rocket man: Half-life 2 episode 2, never have I been so amused and frustrated at the same time...

 

[Xojins]

The one you get for being on a boat with T-Pain. I mean, Moon Shot in TF2. It's very satisfying.

 

[squid5580]

Ghostbusters because the tiles are so sweet. And Matt Hazard because the descripts are hilarious.

 

[Gamer137]

Vesperia Master - Beats Tales of Vesperia 100%.

Impossible without FAQS from, well, Game FAQS.

 

[Markers1713]

Bladder of Steel - Rock Band 2
It's when you play the Endless Setlist 2 (all of the game's 84 songs) in one go without pausing or failing.

I did it on expert, also unlocking the 3 Endless Setlist 2 cheevos.

 

[G1eet]

Weapon X- Beat X-Men Origins on Hard.