9. The Truth - Grand Theft Auto V
The Truth isn't a single side mission in GTA V but actually an entire quest-line, in which Michael finds himself falling in with the Scientology-esque cult known as the Epsilon Program.
The eight missions see players performing an increasingly peculiar series of activities, from searching for aliens to wandering around in the desert for five miles while wearing an Epsilon robe. Oh, and because it's essentially a pyramid scheme, you're periodically "invited" to donate money.
The climactic mission, "Unknowing the Truth," promises players the reward of "limitless knowledge" if they help deliver $2.1 million in "apocalypse funds" to a helicopter, only to end up gifted with a rusty tractor.
Hilariously, though, sufficiently pissed off players can decide to turn on the cause, slaughtering the entire Epsilon security detail and making off with the cash themselves.
Either way, it's a ludicrously enjoyable hour-long detour from the main game, and absolutely vital in its own way.