I was talking with a friend about what sort of setting or milieu I should use to test the early version of the rules for my homegrown RPG, and after mulling it for a while I realized that Star Wars was my general "litmus test" setting for any set of generic or broadly applicable rules. Without speaking to the quality of any particular Star Wars piece of media, I will say unequivocally that Star Wars is the most gamable setting in the pop-geek-culture universe, and for me anyway, if a "generic" rule set cannot do Star Wars, it isn't for me.
So do you have a setting or milieu that is your "litmus test" for any given set of generic/universal/etc rules you want to try? Do you demand a rule set be able to do heroic fantasy or Star trek or Cthulhu investigations?
Yes, I do - but it's for official settings. Does it alter settings to fit the rules or rules to fit the settings. Not interested if it's not tweaking the core to the setting...
IME, GURPS started by altering the rules to the settings, but somewhere in 2nd changed to altering settings to better fit the GURPS rules... And that's about when I started liking it less.
Hero (since 3rd) says to limit the rules for your setting and gives you forms (since 4th) for laying it out... But 2nd and 3rd edition rules had been ported with rules changes for setting to Fantasy Hero, Danger International, Justice Inc., and Star Hero...
Palladium Multiversal is theoretically a tweak the rules to the setting, but not by much. Given the core isn't great, none of it is. Further, it's never directly presented as a universal system.
Savage Worlds is tweak lightly but tweak the rules to the setting. I consider Savage Worlds to be Genre centric to Pulp and semi-pulp. I could see using it for Star Wars, maybe for star trek, and definitely good for Space 1889 (via Red Sands)... but not for classic D&D Dungeon Fantasy. Nor, for that matter, doing hard SF.
GDW's House Engine (used in T2K 2.0, T2K 2.2, DC 1.0, DC 1.1, Cadillacs & Dinosaurs, and T:TNE) was another tweak lightly, don't touch the core mechanics (excepting the edition breaks; 1.0 is T2K 2.0, DC 1.0 and C&D; 2.0 is T2k 2.2 and DC 1.1; 2.1 is T:TNE). I don't mind the 1.0 nor the reduction for terms in skill receipts of TTNE, but I dislike the d20 version for overvaluing attributes. 1.0 undervalues them for skills, as they only factor in as a fractional skill level. Too much aptitude... not enough training. One can, if familiar with any non-GURPS non-Hero version of Traveller, see the Traveller lineage in the GDW house engine, except C&D. Aw hell, more detail: 1.0 uses 1d10 ≤ (Skill × Diffmod), and easy diffmod is 2, normal is 1, difficult is 1/2, and formidable × 1/4 (used only in combat). 2.0 uses 1d20 ≤ ((Att+Skill) × Diffmod) with Simple 4, routine 2, difficult 1, formidable 1/2 and impossible 1/4. In both editions, an attribute gain replaces two skill levels in character gen, and then only as a hobby activity. I consider that 2:1 cost too low for the 1:1 ability. Even given the difficulty shift for unskilled use (effectively halving aptitude), attribute dominates. Further, it was never released as a generic core.
EABA and CORPS are both tweak both setting and rules. Note: I playtest commented supplementary material for CORPS Companion and playtested EABA 1.0. I don't like the changes of EABA 2.0.
2d20 as a system has remained tweak the core to the settings, with separate setting based cores, and hasn't a strong reference edition baseline. I've enjoyed STA, Dune, and Fallout... but the having to remind players of the mechanical differences has been a minor pain. I can see using it for Space 1999, Mutant Chronicles... but I'd not want to do Cyberpunk with it.
Year Zero Engine is clearly also tweak the core to the settings, again with setting linked cores. I find it more flexible than 2d20, tho' for a more pulpy feel, it'd need some heavy tweaks. It's a pretty rough game, with low success odds.
D6 System has proven itself time and again as suitable for action genres and non-action play alike; the original version of the generic engine was not open licensed for commercial use, and saw little uptake, but with the death of WEG, WEG went OGL on the three Genre cores: D6 Space, D6 Adventure, and D6 Fantasy - all three are the same core rules except for the paranormal mechanics; they're all essentially Star Wars 3rd Ed sans the SW IP. Earlier games adapted the whole engine... Herc & Xena was whole dice only, different atts, MIB used different damage mechanics, etc... there was even a count successes variant... one of the DC licensed games.
D&D 3.0: I think it's fine as a genre engine for Dungeon Fantasy, and if well tweaked (True20, T20 Traveller's Handbook) a pretty good system within reason
D&D 5.0: I think it a slightly better game than 3.x for DF as a genre engine, and find the best expression being Pugmire. I find it unconvincing as a multi-genre engine... 3rd party commercial cores are largely tweaking the rules to settings; fan work usually is tweaking settings to the D&D core.
Tunnels and Trolls was treated by a sort-of-licensed 3pp as a generic engine, for doing a space fantasy setting, and another such 3pp for "Gamma Trollworld" - a T&T pastiche of Gamma World. GTW was not too far, but the SF one was not my cuppa...
There was a supers port for The Fantasy Trip. I didn't care for it. There was to be a Sci Fi port... only one adventure was released for it. Fan ports to Traveller were done, they were complex; complex enough I didn't even try to use them.
Consolidated thoughts
I've not opted for ported often; Deadlands Reloaded, GURPS Autoduel 1e... I liked the deadlands port - it felt like an edition boundary simplification more than a port; indeed, Savage Worlds is derived from Deadlands. I, at the time, liked GURPS Autoduel 1e... and hated GAD 2 with its requirement for GURPS Vehicles, a supplement I found excessively detailed in the wrong places. I haven't liked any of the GURPS ports for Traveller - neither the fan one from 1994 nor the official ones.
I've liked many licensed "bend the rules to the settings" approaches, especially Alien, STA, and Dune. And even Blade Runner and T2K 4e.
I'm done with "bend the setting to the rules"... I can do that on my own, and don't like the outcome either way. I want the tweaking to fit done to the rules already. I don't mind if, like Deadlands, it's a supplement of rules tweaks, or, if like Dune, or alien, it's a standalone adapted core.