I have heard GURPS, Cortex Prime, and Fate (plus others) all called toolbox RPGs. And I have seen criticism about difficulty of use aimed at most of them. But which toolbox RPG does the best explanation for newcomers? What is the best toolbox RPG to use first?
And I know the usual advice is to choose the toolbox RPG that fits your campaign concept the best. But maybe that is the best SECOND toolbox RPG to try.
You're missing several currently available: Savage Worlds, EABA, Hero System, BRP, Genesys, Mekton Zeta...
Fate actually is multiple games: Fate Core, Fate Accelerated, and a bunch of adapted core Fate system games.
And some others: Masterbook, d6 System Toolkit, CORPS: Complete Omniversal Role-Play System (2nd ed), Silhouette Core...
They generally fall into three clades:
Stable core with settingbooks: GURPS, BRP, Genesys, Masterbook
Stable core with a lot of bolt on components in core, and sometimes more in supplements: EABA, Mekton Zeta, Hero System, Savage Worlds, CORPS (the skill list is a build your own!), Fate Core, Fate Accelerated.
Build a system from list of components: Fate Core, d6 System Toolkit, Fuzion.
Each clade is very different in how player experience can happen.
A few genre specific games have extensive toolkit elements, too... most notably The Burning Wheel, and ICE's Rolemaster and Spacemaster. Mekton Zeta is Mecha Anime focused, but can be used easily outside that genre, and even points that out.
D6 Space,
D6 Adventure, and
D6 Fantasy are worked genre engine flavors, directly cadet from WEG Star Wars... with some of the
D6 System Toolkit options included and otherwise being tweaked to the 3 genres.
My initial suggestion is one of the D6 genre games, Fate Core, or Savage Worlds, as they're fast to get to play, and the play-loop is simple in all three cases. I'll note that Fate Core has a very strong player-skill component, but it's only a problem if a power player is doing so and stealing all the face time.