Be it a game that's difficult in its entirety, or a particular challenge in a game that you just couldn't complete.

For me, there are three that come to mind:

• Super Hostile: Waking Up (Minecraft Custom Map, Hard difficulty). That damn water section...
• Terraria Zenith Mode. Coming from someone to whom Master Mode is fairly easy, this was rough.
• Calamity Death Mode (Terraria). I beat DoG (this was back when a headshot was an automatic death), but I just could not click with the Yharim fight. I also think burnout was at play here because that was a LONG playthrough.