From Chainmail to D&D Next
GURPS is one of the first truly generic systems, that has lots of great source books. GURPS has become a toolkit from which you choose the options you want to use. Depending on the options you choose a fight with some bandits could last 15 seconds or over an hour. I own a copy of the lite starter rule, which make a great rules light minimalist core. The full system are two almost 400 page books, plus what ever supplement you are using.
GURPS generally stresses realism, so when calculating falling damage use the actual physic velocity equations and you will have to do a square root. GURPS provides you a rule for just about anything you characters might encounter, but using all those rules makes the game very complex.
The system uses 3d6 roll low to resolve almost everything. Combat is a quick attack-defend-damage sequence, with lots of optional maneuvers. Character creation is a point buy system. A GURPS character is made up of three different things: Skills, Advantages and Disadvantages.
GURPS has several good fantasy and magic supplements, that will let you create any fantasy setting you desire.
Play this game when you want to have a generic system that you can play from lite to a juggernaught of rules, with some of the best source books on the market.