For sure Minecraft Modpacks. Can set up factories for stuff you need and leave it running.
Terraria
That's what I played, they already have been mentioned.
Would have loved it to have a Arma 2 Dayz Mod Server when it was on its peak. Get myself enough loot and raid other ppl on the official servers. But there were enough unprotected "hack" servers where you just could hack in your loot without getting banned. Oh Yea was a fun time. No, I never teleported ppl or nuked a whole server. I just got myself back what other hackers destroyed.