Most rare games you own on your Steam account?

I think that Grand Theft Auto I & II are "rarest" in my collection

Breach, MLB Front Office Manager

The Cat Lady.

Only one friend has it, and its because I gave him as a gift.

Darkspore. It was removed from the store a long time ago, a shame really, it's a pretty awesome game.

Onimusha 3: Demon Siege

Vault Cracker.

None of my Steam friends own it.

Neverwinter Nights 2 and Deadpool appear to be the rarest games I own.

Darkbase 01 ;_;

Probably either Deadpool or Fable 3. Although I suspect Deadpool might make a comeback of some sorts once the movie hits just to drain the franchise of some more money.

The Stomping Land HUEHUEHUEHUEHUE.

I think LEGO Star Wars 3 and Saga

I think it's Foul Play and Greyfox:)

I have a few games that were removed and then a few games that none of my friends have

-----REMOVED------
metro last light
spacewars

-----FRIENDS DONT HAVE-----
tomb raider III
tomb raider: the last revelation
thief gold
supreme commander
mini ninjas
lichdom battlemage
the last remnant
kane & lynch 2 dog days
hitman: contracts
The guild II
deus ex: invisible war
Arma tactics
Anachronox
Cubot

EDIT: I have 83 friends, a few of which have over 1000 games :)

I found one game my friends don't have: Adventure Manager
and one game that was removed: Mana Crusher

I didn't look terribly hard, but I found a few things. c:

None of my friends have
Toren
Fallen Enchantress: Legendary Heroes
Monstrum
Oceanhorn: Monster of Uncharted Seas
Republique Remastered
Sanitarium
The Incredible Adventures of Van Helsing II
Vulture for NetHack

And also Grim Dawn. The only person on my friends list who has it is my boyfriend, and I bought his copy when I got mine to make him play it with me.

Pillar = Owners: -394 - 1,218
DRAGON: A Game About a Dragon = Owners: 337 - 1,835
Celia's Quest = Owners: 1,425 - 3,735
Countless Rooms of Death = Owners: 1,011 - 3,063

Arms Dealer

-Aces Wild: Maniac Brawling Action!
-Batman: Arkham Knight (it came with my GPU, don't judge me)
-Crypt of the NecroDancer
-Cubetractor
-Day of Defeat
-Deathmatch Classic
-Half-Life 2: Lost Coast
-Half-Life Deathmatch: Source
-Left 4 Dead 2 Beta
-Marine Sharpshooter II: Jungle Warfare
-Realms of the Haunting
-Ricochet
-Rocketbirds: Hardboiled Chicken
-Space Hack
-Team Fortress Classic

I'm not sure it's that rare as far as its presence in people's libraries, but it has been removed form the store, so you can't get it anymore: Clive Barker's Jericho. Too bad it's not very good. But god news for those who don't have it: you aren't missing much.

Gothic 1-3 (Universe Edition)

Among my top ten favorite games, too.

Rarest is very likely now ended closed alpha of a unreleased game covered by NDA, so I can't say it's name.
It's still in library and can be downloaded & launched, but can't be played because of server side check :(

Second rarest likely, Rain marginal, 400 owners according to Steamspy :P

Mafia.

Dirt 2

Ironcast - non of my friends have it*, and I'm really suprise of that, because this game is AWESOME!!! This is only game where I have all achievements.

P.S. Sorry for my terrible english :)

*(or maybe because I heve only 16 friends on Steam ;D)

Hmm... Maybe Under the Ocean, I dunno.