Since direct 1.21.1 clients can be buggy, many players use "EaglercraftX" (1.8.8) to connect to servers that utilize plugins to simulate or allow play on 1.21 worlds. ShadowNetworkDevs/Eaglercraft-1.21.1-Server - GitHub
The search term appears to contain a typo or shorthand. It most likely refers to Eaglercraft version 1.21 or Minecraft 1.21 features (Tricky Trials update), combined with “1 best” meaning “the best version 1.21-like Eaglercraft experience.” eaglercraft 121 1 best