A week ago i somehow managed to buy fully working HP Proliant DL360 G7 with 2x Xeon X5660 and 48GB RAM for 5 euro, and now i dont know what OS to install, because Windows Server 2012 R2 (which i already installed) dont have enough important drivers to work correctly (like ATI ES1000 driver), about Windows Server 2008 R2 i am unsure, and i havent any experience with Linux server operating systems. What is the best system now for that old server?
Depends entirely on what you want to do with it. Mostly storage with some containers and a little light virtualization? TrueNAS Scale. Mostly virtualization with less robust storage features? Proxmox.