An OP would be nice, but it would take some time to write. Here are some basic facts, I guess (and keep in mind I'm not a primary source; I got this information off the Internet - in chronological order).
SpaceBattles (SB), circa 1998
The original. Founded in 1998, or something like that. Talks about sci-fi and worldbuilding, and has interactive quests (choose-your-own-adventure stories). As it is older than SA (but only by a year), expect many a lolcow. If you see someone with a pre-2000 join date, they are basically to be treated as deities (assuming they haven't been banned). Also became a lefty shithole. Many banned users on SB went to QQ or SV (see below).
Questionable Questing (QQ) (c. 2012 - 2013)
Founded sometime in 2013 (either January or February), but may have been founded as early as 2012. A smut-filled SB fork for people who are too NSFW for SB or SV (see below). Most of what you see is smut, although you need an account to see the NSFW stuff at all (they do have SFW quests and stories, though). Has almost no rules except "don't post anything illegal" and "don't post about politics" so there's plenty of quests from sick creeps there and banned users on SV sometimes post there to bitch about SV or post their shit. Despite its creep factor, it's the only one of the four sites on this list that isn't disgustingly in-your-face political (by virtue of disallowing politics entirely).
Sufficient Velocity (SV) (April 21, 2014)
Founded April 21, 2014 over the concerns of a user named Athene being banned. Several banned users on SB founded SV, two of whom were lawyers (Lord Squishy and Ford Prefect). They also ended up being a lefty shithole, except they have a kangaroo court (because it was set up by lawyers). They have rules which ban rageposting against fictional characters, an exhaustive amount of fake "legislation" (no, seriously, look at the Administrative Tribunal) and they're your typical lefty shithole.
The Sietch (TS)
Founded in 2018. Check the post above. tl;dr it's a dead lefty righty shithole.
That's all I can think of. Someone with more expertise could chip in and make a much better OP.