My pick would be "basically every 90s PC game" but that would be cheating. So my answer is all the Mechwarrior games.
I heard nothing but hype for these games for years, only to actually play them and find them lacking. The deep lore? Not in the games. It's in several novels that were never released in the UK as far as I can tell. The in depth customization? All the mechs are the same aside from tonnage and hitbox (except in MW4 and MW5 which fans complain about). The tactical simulation gameplay? Biggest mech wins. Get close for short range weapons, or keep your distance and spam long range weapons.
Even the fans don't seem to be fans of the games. They name drop story lines like "the word of Blake", "the merik civil war" and "the dark age", or mention characters like Dominic Payne or organizations like The Kell Hounds, but when I ask for details none are provided.
Sin episode 1 was way better than Half-Life 2 episode 1, but the latter overshadowed the former. I guess Sin was always getting screwed by Half-Life release dates.
Honestly, Mechwarrior/Battletech's lore got retarded after MW2. Basically imagine the Chinese classic Romance of the Three Kingdoms, if it had a crack-baby with Dune, and then was raised by suits that forced it to keep going long past any logical stopping point just like various Marvel and DC properties because it was profitable.
Early on the story was just... there's these five great houses and they're fighting for control of the galaxy (called the Inner Sphere, because anything outside IS is regions of space nobody has explored and come back from). For a long time the novels were basically micro-stories about how some merc team or person played a role in this large-scale conflict. Imagine those parts of Romance of the Three Kingdoms where some one-shot character wins an important battle, if all those parts were turned into full novels.
(And if you've never read romance of the three kingdoms, I recommend
this translation. Seriously actual good literature is better for your brain than all this corporate shit... even if corporate shit is fun. EDIT: If you go for a Kindle Edition, make sure it specifically says "translated by Moss Roberts." For awhile Amazon was selling a machine-translated version that was basically unreadable gobbledegook).
Then suddenly they did this storyline called "The Clans."
Backstory: The Inner Sphere used to be united under something called the Star League, but it couldn't keep its shit together and split into the five territories. When this happened, a dude named... Kerevsky or something (too lazy to look it up) basically said he was tired of dealing with Inner Sphere manbabies and took whoever would come with him into the unexplored regions of space. This turned out to be a good call--they set up this radically different sort-of warrior culture caste society where lots of people are cloned from DNA of great leaders and getting highly regarded enough to have your DNA stored (called getting a "bloodname") is highly prized. There's sixteen clans, each named after an animal, and each having different goals and values (though there are some major things ALL the clans agree on).
Then the clans decided to invade the Inner Sphere.
Thing is, IS was fucked--the long never-ending war had actually caused their technology to stagnate and even backslide (in this setting, salvaging undamaged mech parts is highly prioritized because they actually can't manufacture replacements, and a lot of roleplay sessions had battles motivated by "Two opposing armies have found an untapped supply of unused mechs and spare parts" type events). The Clans just had better everything.
Somehow though the five houses managed to pull together and beat the Clans off. How? I dunno--I haven't read those novels yet.
After that though it just keeps going in a circle... for awhile there's a new Star League, but the current leaders are just as much manbabies as last time and it winds up splintering. As for things like Word of Blake, I have no fucking clue--I haven't read those novels yet, and to be honest I might lose interest before I get that far.
Mechwarrior 1's plot is entirely incidental in the grand scheme of things--just some small duchy-boy reclaiming his meaningless slice of pie. Mechwarrior 2 is based off an event called the "War of Refusal" where basically two of the Clans went to war because one accused the other of.... something. Here,
this guy probably gives a better sum-up than I could. I have no idea about the later Mechwarrior games.