I have a whole wall of tabletop RPGs, but most are collectible / mined for ideas.
For systems, Gurps (and DFRPG) is my goto system, but I also play somewhat regularly :
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Abstract Dungeon : a great universal game well suited for short games or testing new concepts.
In my latest Gurps campaign (based on THS/the expanse), we ran the first session using Abstract Dungeon with hastily sketched characters, and the players and I then created their Gurps characters based on that experience.
***Call of Cthulhu (Chaosium) when I play a Call of Cthulhu game. Just because.
I do think Gurps is actually much better for low powered horror/investigation game, but I have been using CoC for so long it is second nature for me.
If it is not Cthulhu Mythos related, I switch to Gurps (or AD)
***I recently got convinced to run a few shadowrun game, and I discovered
Shadowrun Anarchy.
After being driven away from the game by the 4th and 5th edition that were not for me, Shadowrun Anarchy proved to be great at quick, fast paced runs.
The french translation + the fanmade french companion add a lot, the US edition need some work. (I have both)
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Green Wall Mall is a recent addition to my library, and already proved to be great for demo games and for games with teenagers.
***King Arthur Pendragon. I played it a lot way back, I am currently collecting materials to attempt "The Great Campaign" ... just need the players :(
The rules match the setting well enough, and the main accent is on estate/political/family/duties management anyway.
***Mage the Ascension ... although Gurps with RPM is getting close. If I were to play again, I do not know wich system I would use nowadays.
And so many others.
For me, the system is often secondary. I can and will happily use either the original system, or an universal one that I already master.
Some exceptions with specific games where the system and the universe are really woven together and using the original rules add a lot to the feeling of the game (like the French game "Rêve de Dragon" and it's magic system, or original Deadlands with the poker decks and chips, or Pendragon with all the rules for virtues and passions, seasonal growth and estate management)
But mostly,
**if I play in a published setting, I will likely use that game system (with exceptions for systems I don't like or feel hard to play (such as Thoan, AD&D, Gumshoe, anything "powered by apocalypse", shadowrun 4,5,6, Nephilim, The Expanse ... for those, it is Gurps - except Nephilim, I haven't found the right set of rules for it yet)
**if I play in a setting of my own, I will use Gurps or Abstract Dungeon.
I tried Savage World as a player, and wasn't convinced... too complex for a simple game, to simple for a complex game ? But I confess I did not explore it in depth.
Never tried Fate. I got Dresden File, but I ran the game inspired by it in Gurps.
I used to play (A)D&D about 30 years ago. I switched to Rolemaster/Call of Cthulhu/Gurps/Traveller quickly, and I haven't ran a D20 system game since.
Today if I run hack&slash fantasy, it is in DFRPG.