Best coupon ever existed/crafted in Steam?

There's been a bunch of 90% off coupons. I'm not sure it gets any better than that. As for which 90% off is the best, that would depend on which ones have been spotted in the wild so far. I think the Portal 2 90% off coupon would be awesome, since that's a tremendous value for $1.99.

I've seen 90% off:

3SwitcheD
Iron Sky Invasion
Serious Sam 3
Two Worlds (1 or 2, I can't remember)

and some others

Skullgirls 90% when the game was in preorder.

Well, I've only got 90% off Portal 2 and Two Worlds :P

Portal 2 90%

Gold coupons FTW

HL2 90% off. Got HL2 for 70 cents then sold the cards and made more than that.

I had a L4D2 90% off coupon, traded it for a pretty good game.

90% off Serious Sam 3 for 1 scrap.
90% off Portal 2 for 1-2 keys o.O