Without reading anyone else's posts, I think free-for-all with real consequences is the best. No MMORPG games that I know of have real consequences, just artificial constraints. Real consequences means that the "carebears" can have an intrinsic value to "society" such that anyone who kills them really becomes an "outlaw". Real consequences means that, for instance, you can't use the starports in SWG to escape from retaliation if you just wiped out the local constabulary.
The trouble with most FFA PvP is it does not pretend to model a real society but a megalomaniac fantasy centered around "old number one". That's fine if you are making a shooter fantasy game, but doesn't fit what players with social RPG prospects want to pay for. As soon as you have a society, there are implied rules for interacting with members of that society. If the society happens to be violent, there are rules for who you can and cannot do violence unto, and under what circumstances, without consequences. Modeled RPG societies should attempt to model the consequences, instead of just the constraints that the consequences are supposed to enforce.
I can think of two main reasons why the kind of FFA PvP I'm talking about has never really existed. First, the split between "carebear" and "FFA" is adequately managed by different rulesets, to a level sufficient to provide a market presence for both niches. Second, it's difficult and expensive to create a good RPG societal model with consequences for crime that is appealing enough to all players. What these two points mean when taken together is that any game that tried to create a "realistic" societal model might find itself without players, since its FFA and carebear contingents might each desert to games that better fulfill their respective fantasies.
Early SWG was actually a good step in the direction I'm talking about. It did not go far enough, because travel was much too easy, and some consequences needed to be more severe and widespread. For example, battles between partisans in cities should have met with overwhelming NPC responses, according to the current partisan alignment of the city. For another example, typical PvE actions like killing every yellow name in sight should have met with overwhelming NPC response. Faction should not have been something which was easily recovered when lost, just by killing an equivalent number of undesirables; SWG just followed other PvE games in this respect, when it should have invented its own rules.
The later "combatant" and "special forces" rules in SWG were kind of laughable to me, and contributed a lot toward dissolving my "suspension of disbelief" in that game's context, in spite of the fact that I played as a complete non-participant in the GCW. Unfortunately, I'm sure that better, more immersive, more consequential rules would have driven away many more players, both PvPers and PvEers. They were primarily interested in getting their shooting thrills without putting up with appropriate societal consequences.
I don't want to go on with more details, because I think you can all see what I mean.
Edit: That being said, I voted for total FFA, with the caveat that real consequences are needed, and NPC's should enforce those consequences. Faction-based is my second choice, mainly because factions are an easy way to keep from accidentally killing someone you don't want to kill, and not having to watch your back constantly. At the same time, the way faction is usually implemented is wimpy, and sucks worse than FFA without consequences. I only play PvE, because I'm just not up to the kind of work that's necessary to enjoy PvP as it's currently done. The poll was about PvP, so I'm taking it to mean what kind of PvP that I would consider playing.