Blockland is good because you can build a whole lot of things, from detailed builds to large-scale cities.
Minecraft is good, because it updates from time to time, adding lots of rad features and stuff (such as the 1.14 update), and also a limitless amount of possibilites (not really, though).
Roblox is good, because of it's good variety of styles games (places) can have, like Phantom Forces or Flood Escape 2, for example; not to mention the "flexibility"(?) of the scripting, which is based on Lua. HOWEVER, it is infested with bots that spam robux scam links that steals the players' accounts, which are mostly kids, and also the filtering system that forbids us from swearing because kids.
Honestly, Roblox is the least good of the three games, but it has it's own good things.. such a good game going to waste