There are many answers to this question, depending on what type of game I am running.
Heroic action/adventure or Space Opera/Science Fantasy goes to WEGd6, now freely available online.
Perfect for that Star Wars game you are wanting to run, or that 1920s-1940s against the Nazi's game. Also works extremely well with a lost world type of setting set in the jungle.
Sword and Sorcery definitely goes to the Talislanta system for its simplicity and magic system.
The ease of creating spells, the elegance of the system...absolutely adore it. I have been using the Talislanta system for all of my fantasy games since the release of 3rd edition D&D.
SLA Industries has a system that I have always found worked perfectly for the game itself, and I have always felt the same in regards to D&D, AD&D and now D&D 5th edition.
I am of the persuasion that the system has to fit the setting. SLA Industries and the 3 versions of D&D I like have systems that fit those settings, and no other system is going to work.