Best game for 20 euros?

Erm Chivalry its about 10 dollars right now on Greenmangaming use the promo code for cheaper. Combat is really good.

There aren't really great deals atm, in my opinion.

But if you are desperate to buy Steam games now, I suggest you buy Krater on GMG, Indie Gala Bundle and Bundle Stars

Maybe Sanctum 4 Pack if you have friends that are into Tower Defense games (there's a Demo you can try first)

Worms Revolution