Out of all of the systems I've ran and played, I probably have the most to say about GURPS. It stands for Generic Universal Role-Playing System, and it's gotten a bad reputation in some online circles as being too convoluted and requiring advanced mathemetic knowledge to play. I'm not a math expert and I'm not particularly good at keeping track at a lot of moving parts and I manage to play and run it just fine, it's not that complicated. It revolves around a bell curve of three six-sided dice, which gives an interesting spread of probabilities- while also letting characters who invest a lot into certain skills flex those skills without goofy outcomes like automatically failing every time you roll a 1.
The modularity of GURPS allows for a wide array of campaign and player character concepts to be ran with little to no hitches in the system's mechanics, from demon-binding occultist cowboys to super powered welfare recipients fighting politicians to the death (I actually ran this one.)
My favorite aspect of the system is by and large the fact that it's a toolbox where all the parts can fit together with relative ease, along with the more in-depth options for combat if you use books like Martial Arts for some serious high stakes duels between skilled combatants. Facing and how much you're weighed down and a myriad of other circumstances determines just how well you can dodge, block and parry attacks, leaving players to try and balance just how kitted out they are against being able to move around effectively.
Fnord.