What game should I buy?

Well FTL is on deal.

If you don't have a lot of money, just wait for a good sale.

9x Fortix

Binding of Isaac + DLC.

FTL

wait for a sale and buy stuff you actually want

There are over 1000 Games under 10 euros..

you can buy FTL or ISAAC as a Gift and make a Giveaway^^

And you can check in Steam too, just get the games in order by price so you can check the ones under 9€.

Don't ask this question if you don't hesitate between some games.

If you don't want at least one particularly, then just keep your money for now.

http://www.steamgamesales.com/

You can buy Serious Sam 3 for 6$ (or 6 EUR) with a 85% discount coupon from AlienWare
http://eu.alienwarearena.com/deal/85-off-serious-sam-3-bfe

Id invest it, because with each passing day your euro gets less value due to inflation