Taking a note from The Hacker, Blockland is (pretty much) the only game of it's kind, and of it's time. It's played/already plays role as a Sandbox, an FPS, a Roleplay, Parkour Challenge, a Simon Says game, and there are plenty more things I didn't list it has been.
Blockland, by far, has the most playtime on my scale of greats, with about 5-6 years.
Blockland is also technically my "first" game I've ever owned.
wow. its cool to know others would put blockland on yearly playtime. its taken me 7 yrs to do what pretty much feels like everything.