1€ on Steam

You'd probably be better off waiting for a good 1$ tier bundle, but assuming this money is in your Steam wallet I have to say that I have troubles finding a good game for 1€. Currently browsing through Steam games and if you would add another euro or maybe just 75 cents I could recommend Deus Ex, for example. :D
Oi, Psychonauts costs 99 cents!

geometry dash or almost super meat boy

Edit: you could probably get to 2 euro by selling all the summer sale trading cards every day

Depends on what you think are good games. Just look at the good games for that price and see what you like.

This
Maybe This
Or this
This as well
This is not bad
Not bad either
Then there's this
And also this
As well as this
All these "this" are starting to look alike
This to break from the puzzle ones
This too
This seems interesting
This is episodic, beware! Only Episode 1 = 0,98

rating 75%+, ordered by price, starting at $0.49
https://steamdb.info/sales/?merged=true&min_rating=75

takes some time to load the list

Psychonauts.

postal 2

Arma: Cold War Assault

Get 50 more cents and get how to survive storm warning edition.

If you're in the mood for something a little bit weird and different, try Verde Station. It's short, at about an hour long, but it's still a hell of an interesting hour. Just don't read anything about it before you play! Best to go in blind.
Like others have mentioned, Psychonauts is really good, too. As is Stacking, Costume Quest and Lili: Child of Geos.
Hope that helps : )

Attack of the Labyrinth + can be fun. But for most it seems rather generic. But then again it is at sale for 1 € and I think you get that money's worth.

Outland

Orion

Maybe BloodGate if you're fine with match 3 type of games. You can still clearly see that it's a mobile port, but all microtransactions have been removed and it's alright for that price.

Stacking is a lovely adventure with a gorgeous style and unique mechanic. I recommend it to anyone who likes the genre, particularly as it's 90% off in the sale right now.

One Way Heroics

Postal 2 or the Guild 2: Renaissance

Hero siege for 0.59€

http://store.steampowered.com/bundle/246/

Pack of four 9/10 sci-fi RPGmaker games for 0.6€ or even less if you own something already.

thks ( i bought Blood gate) =)

http://store.steampowered.com/bundle/575/ Civilization V: The Complete Edition, ONLY CAD$0.90.

my picks for you

Hero Siege 0.59€
Its a great and really fun game.