"What pseudo or alternate history RPG have you enjoyed most? Why?"
You know what? I think I'm going to deviate again from standard RPGs, as I just talked about Call of Cthulhu twice, as both my favorite horror and historical tabletop RPG. Instead, it's time to go out on a limb and pitch a setting creation for my own later use.
A couple years back now, I bought the BESM (Big Eyes, Small Mouth) 2nd revised edition core ruleset, because I like anime, and I like gaming. It's been sitting on my gaming shelf since I bought it. I've read through it a bit, but haven't ever really sat down to write anything for the game, or try to run it for friends. This is obviously a travesty, as there are myriad amazing anime series that are just waiting to be plundered for their goodness.
One of those is Valkyria Chronicles, an alternate-universe take on World War II where a small, neutral, resource-rich Europan (not European) nation called Gallia must defend itself from being absorbed by a more powerful neighboring country, called the Empire. The resource, instead of petroleum, is called Ragnite, and is a kind of unobtanium, capable of being used as a fuel for tanks, an accelerant to the healing process, and a key component in weapon function.
Also, because it's an Anime, there are magical women from an ancient stock of Valkyrur that can turn blue and wreck absolutely fucking everything. Obviously, there aren't very many of them.
Converting Valkyria Chronicles to BESM solves a few innate problems with the system. First, it's a specific genre of anime. Characters created in the system have the restrictions of being soldiers, but the lore of the game allows for a multitude of types of soldiers to be created, and latter games add even more insane careers to focus on. VC1 played it relatively safe with Scouts, Shocktroopers, Lancers, Medics, Engineers, and Snipers. VC2 added Fencers and Armored Techs. VC3 upped the crazy by sticking cannons on giant swords. So, depending on your (and your players') familiarity with the series, you can be as conservative or insane as you like with the tech.
Second, it reduces a good number of character options for the system. Magic doesn't really exist in the universe, neither do psionics or superheroes. There are the Valkyrur, but those are super-rare and can easily be nixed by a prudent GM. It establishes a low power level for characters but with the knowledge that your opponents aren't going to be crazy strong either.
Lastly, being based on an existing anime property, there's already a hell of a lot of lore that isn't present in the games existing on the internet because of zealous fans putting things together or the developers releasing their notes.
With all the projects I've taken on this year, I'm hoping I won't get burnt out before attempting this one. Fingers Crossed.