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Even though it's a free mod with bugs and glitches, I rate this open-ended sandbox game as my #4 favorite game of all time. You can do whatever the hell you want. It's a virtual world with very few rules, and you can custom build, craft, or modify buildings with very little restriction, even pre-built game buildings. Almost everything can eventually be destroyed, except safes, which keeps the focus on multi-player interactions rather than an npc grind.
This is the video that got me started a couple of months ago. I had Arma 2 which I played maybe once or twice and had no idea such mods were available:
For those that have not tried it yet, it's a free mod for Arma 2 Combined Operations that plays like an MMO, but with no subscription fee. Right now you can get Combined Operations for only $6.23: https://store.bistudio.com/arma2-games
I recommend using DayZ Launcher to automatically install and update DayZ Epoch and connect to servers.
I don't recommend the DayZ standalone that is being created, it sucks in comparison to Epoch. Maybe it will get better, but I doubt it. I think it's being developed more for making money rather than purely out of passion like DayZ Epoch.
Probably the biggest improvement that could be made to the game, aside from cleaning up the clunky interface and bugs, is to make it an actual MMO with thousands of players per server instead of 50-100. But it's still a lot of fun even with only a few dozen online at once. Arma 3 Epoch is also in development, which should be an improvement.
Here is the Epoch development website: http://epochmod.com