Now for the factions themselves...
Recommended: University and Morganites
Considerably Harder: Peacekeepers
Very Difficult: Hive, Spartans and Gaians
“Playable”(?!...): Believers
Now for the reasons behind my rationale...
University: These guys are obvious. Their extra RESEARCH, will never disappoint, especially in the One City Challenge, where it becomes so much more vital. Yes, there is a PROBE penalty, but countering that, can be done, even without Fundamentalist Politics (these guys’ Aversion). Yes, this makes Miriam almost impossible to appease, but hey, at least , you can, somewhat, keep Yang, off your back. And even if, war, against you, is declared, your truly superior, tactics and equipment, will just, make steam rolling, the enemy, an absolute breeze. So as always, these folks are still one of the best factions, if not the best faction, to play as especially, in the One City Challenge.
Morganites: Energy powers your faction’s (anything), including its Labs, Psych and Bank Account; and these guys gain extra Commerce and Energy. Yes, they cant pop-boom in the early game (their default pop-limit sucks, and their Aversion is Planned Economics), but their extra economic power, will more than make up for that. The best part?: having only a single city for the duration of the game, will make Economic Inefficiency/Bureaucracy, a thing of the past! Yeah, they have a SUPPORT Penalty early on (followers have high expectations), but only remember to stay in Frontier Politics, in the early to mid game, and you’ll be totally safe. Yes, your signature Market Economy, will provoke, both the Gaians, and the Wild Mind Worms, but strike first, at the latter and appease the former, and you’ll be fine; and even if, Deirdre, does, declare war, on you, the fact, that, she, suffers, from, MORALE, and POLICE Penalties, will usually, make her relatively easy, to, deal, with. Yes, the Spartans and the University will become angry with you for selecting Wealth Values (in fact, Wealth Values, are Sparta’s Aversion), but usually, you can, just, throw some of your profits and/or technologies at them, in order to buy yourself some more time. And even if war gets declared against you, then the Spartans, have trouble producing, in high numbers, of their otherwise well trained forces, and your patently ridiculous Bank Card, lets you exploit the University’s vulnerability to Probe Teams, squarely, to your advantage, too. I highly recommended, these saps, just, for the sake, of, this very challenge.
Peacekeepers: Your Workers, are literally the “salt of your faction”, whilst your Talents form the veritable “cream of the civilian crop”. And not only can these fellows pop-boom early on... they also have a rather high default pop-limit! Best of all, their supreme lack of restrictions on personal freedoms gives them an extra Talent for every four Workers (Civilian-Class Elite)! Yes, they suffer a slight Economic Efficiency Penalty at the start (U.N. Styled Bureaucracy). As always however, the One City Challenge makes this problem basically irrelevant. And yes, the extra Council Votes, mean nothing, in the One City Challenge. The Diplomatic Victory however, has been disabled, so this penalty, is also made, a non-issue. Yes, they can’t select the Police State Political System, so they can’t suppress Drones via extra “cops” until the days of Future Society. However, those, auto-extra Talents, will more than make up for that, very early on. Yes, Miriam and Yang will both be almost impossible to keep quiet, but throw your techs and profits at them, or else stay solely in Frontier Politics, if, need be, and, you should be OK. Overall, these fellas, are great for both newbies and pros alike, no matter what the play-style (OCC, “other challenge” or even casual).
Recommended Victories in the OCC...
University and Peacekeepers: Scientific
Morganites: Economic and Scientific
Theoretically Possible/Potential Alternative...
University: Conquest
Feasible(?)...
Peacekeepers: Conquest
Extremely Unlikely...
Morganites: Conquest
University and Peacekeepers: Economic
Hive: Sadly, these fellers, suffer, from a penalty to ECONOMY (little to no personal freedoms) by default. They also can’t use the pop-boom tactic early on (they, utterly, abhor Democracy). However, they do get a free Perimeter Defense at the start, which can help if war gets declared against you. Yang’s suppression of the very worst of Bureaucracy, also lets him choose Police State Politics and/or Planned Economics without, any penalties. Yes, this provokes, both Morgan and Lal, as well as, both Deirdre and Miriam, to anger. However, Morgan suffers from an absolutely crippling POLICE Penalty from his beloved Market Economy, so he, can’t even, touch you, without, Probe Teams. His SUPPORT Penalty, does not help his silly case either. Deirdre’s MORALE Penalty, makes her relatively easy to deal with as well, as, can also her POLICE Penalty. Miriam, can only be a true threat, if, either 1) she strikes first, which, can typically, be, avoided, or 2) she, has, high, level Weapons, technology, which, BTW, she, never will. This, makes, only Case #1 an issue in ever having, to, deal, with, her. Meanwhile, Lal would be a threat if he was actually militarily inclined, which BTW, he isn’t. Yes, the Hive gets a bonus to INDUSTRY (serfdom), which lets them stack those much-needed Projects more easily. If Projects, are, your true biggest affinity, then, go ahead, and, select, Yang; but otherwise, please, do yourself a favor, and stay away from him, for, the One City Challenge.
Spartans: This faction, has all, of the upsides that Miriam, has penalties in. They, can select anything, but, Wealth Values. They are, already well trained, and, well armed (MORALE and POLICE Bonuses). However, they, do, tend, to place, relatively, low, priority, on, extravagant weapons, and, tactics (INDUSTRY Penalty).